I can't open that file in Excel to see how many rows it has, but readcell reads 2886 rows, the first 5 or 6 of which look like the header.
C = readcell('n_fot2017-01-12.xls')
C = 2886×12 cell array
  Columns 1 through 7
    {'Report template (Ra…'}    {[01-Jan-2017 00:00:00]}    {[<missing>        ]}    {[<missing>        ]}    {[<missing>        ]}    {[<missing>        ]}    {[<missing>        ]}
    {'Report template (Ra…'}    {[01-May-2017 00:00:00]}    {[<missing>        ]}    {[<missing>        ]}    {[<missing>        ]}    {[<missing>        ]}    {[<missing>        ]}
    {'Report template (Pr…'}    {[22-Sep-2017 19:47:43]}    {[<missing>        ]}    {[<missing>        ]}    {[<missing>        ]}    {[<missing>        ]}    {[<missing>        ]}
    {'Time series product…'}    {'Kärnkraft prod. per…'}    {'Gasturbin/diesel…'}    {'Vindkraft prod. …'}    {'Netto Exp/imp. p…'}    {'Ospec. prod. per…'}    {'Total prod. per …'}
    {'Numeric time series…'}    {'M273_x_SWE_x_x_x_1'  }    {'M275_x_SWE_x_x_x…'}    {'M272_x_SWE_x_x_x…'}    {'M645_x_SWE_x_x_x…'}    {'M270_x_SWE_x_x_x…'}    {'M500_x_SWE_x_x_x…'}
    {'Numeric time series…'}    {[<missing>           ]}    {[<missing>        ]}    {[<missing>        ]}    {[<missing>        ]}    {[<missing>        ]}    {[<missing>        ]}
    {[01-Jan-2017 00:00:00]}    {[          9.0744e+03]}    {[           0.8574]}    {[       3.6378e+03]}    {[      -2.9196e+03]}    {[           0.9204]}    {[       1.6256e+04]}
    {[01-Jan-2017 01:00:00]}    {[          9.0747e+03]}    {[           0.8508]}    {[       3.4295e+03]}    {[      -2.9203e+03]}    {[           1.1083]}    {[       1.5964e+04]}
    {[01-Jan-2017 02:00:00]}    {[          9.0749e+03]}    {[           0.8447]}    {[       3.2900e+03]}    {[      -2.9965e+03]}    {[           1.0244]}    {[       1.5817e+04]}
    {[01-Jan-2017 03:00:00]}    {[          9.0774e+03]}    {[           0.8266]}    {[       3.0532e+03]}    {[      -2.8781e+03]}    {[           0.9999]}    {[       1.5543e+04]}
    {[01-Jan-2017 04:00:00]}    {[          9.0780e+03]}    {[           0.8007]}    {[       2.9322e+03]}    {[      -2.8557e+03]}    {[           0.8734]}    {[       1.5481e+04]}
    {[01-Jan-2017 05:00:00]}    {[          9.0799e+03]}    {[           0.6707]}    {[       2.7872e+03]}    {[      -2.6600e+03]}    {[           0.6983]}    {[       1.5365e+04]}
    {[01-Jan-2017 06:00:00]}    {[          9.0826e+03]}    {[           0.6390]}    {[       2.6617e+03]}    {[      -2.5739e+03]}    {[           0.0041]}    {[       1.5496e+04]}
    {[01-Jan-2017 07:00:00]}    {[          9.0827e+03]}    {[           0.5710]}    {[       2.6110e+03]}    {[      -2.3826e+03]}    {[           0.6768]}    {[       1.5607e+04]}
    {[01-Jan-2017 08:00:00]}    {[          9.0827e+03]}    {[           0.5523]}    {[       2.5535e+03]}    {[      -2.0771e+03]}    {[           0.6103]}    {[       1.5663e+04]}
    {[01-Jan-2017 09:00:00]}    {[          9.0847e+03]}    {[           0.6233]}    {[       2.3920e+03]}    {[      -2.0789e+03]}    {[           0.1009]}    {[       1.5974e+04]}
  Columns 8 through 12
    {[<missing>        ]}    {[<missing>        ]}    {[<missing>        ]}    {[<missing>        ]}    {[<missing>        ]}
    {[<missing>        ]}    {[<missing>        ]}    {[<missing>        ]}    {[<missing>        ]}    {[<missing>        ]}
    {[<missing>        ]}    {[<missing>        ]}    {[<missing>        ]}    {[<missing>        ]}    {[<missing>        ]}
    {'Solkraft prod. p…'}    {'Vågkraft prod. p…'}    {'Summa förbr. per…'}    {'Övr.värmekraft p…'}    {'Vattenkraft prod…'}
    {'M301_x_SWE_x_x_x…'}    {'M302_x_SWE_x_x_x…'}    {'M501_x_SWE_x_x_x…'}    {'M274_x_SWE_x_x_x…'}    {'M271_x_SWE_x_x_x…'}
    {[<missing>        ]}    {[<missing>        ]}    {[<missing>        ]}    {[<missing>        ]}    {[<missing>        ]}
    {[           0.0359]}    {[                0]}    {[      -1.3339e+04]}    {[       1.1257e+03]}    {[       2.4158e+03]}
    {[           0.0353]}    {[                0]}    {[      -1.3047e+04]}    {[       1.0765e+03]}    {[       2.3812e+03]}
    {[           0.0262]}    {[                0]}    {[      -1.2824e+04]}    {[       1.0955e+03]}    {[       2.3551e+03]}
    {[           0.0196]}    {[                0]}    {[      -1.2668e+04]}    {[       1.1423e+03]}    {[       2.2680e+03]}
    {[           0.0181]}    {[                0]}    {[      -1.2629e+04]}    {[       1.1585e+03]}    {[       2.3110e+03]}
    {[           0.0174]}    {[                0]}    {[      -1.2708e+04]}    {[       1.1691e+03]}    {[       2.3275e+03]}
    {[           0.0191]}    {[                0]}    {[      -1.2924e+04]}    {[       1.1955e+03]}    {[       2.5557e+03]}
    {[           0.0235]}    {[                0]}    {[      -1.3227e+04]}    {[       1.1842e+03]}    {[       2.7278e+03]}
    {[           0.0258]}    {[                0]}    {[      -1.3588e+04]}    {[       1.1659e+03]}    {[       2.8600e+03]}
    {[           0.1066]}    {[                0]}    {[      -1.3897e+04]}    {[       1.1959e+03]}    {[       3.3006e+03]}
Evidently, readtable is interpreting 5 header rows and putting the remaining rows into a table with 2886-5 = 2881 rows.
T = readtable('n_fot2017-01-12.xls')
Warning: Column headers from the file were modified to make them valid MATLAB identifiers before creating variable names for the table. The original column headers are saved in the VariableDescriptions property.
Set 'VariableNamingRule' to 'preserve' to use the original column headers as table variable names.
Set 'VariableNamingRule' to 'preserve' to use the original column headers as table variable names.
T = 2881×12 table
    TimeSeriesProductInstance_Beskrivning_    K_rnkraftProd_PerCO__SWE___    Gasturbin_dieselProd_PerCO__SWE___    VindkraftProd_PerCO__SWE___    NettoExp_imp_PerCO__SWE___    Ospec_Prod_PerCO__SWE___    TotalProd_PerCO__SWE___    SolkraftProd_PerCO__SWE___    V_gkraftProd_PerCO__SWE___    SummaF_rbr_PerCO__SWE___    x_vr_v_rmekraftProd_PerCO__SWE___    VattenkraftProd_PerCO__SWE___
    ______________________________________    ___________________________    __________________________________    ___________________________    __________________________    ________________________    _______________________    __________________________    __________________________    ________________________    _________________________________    _____________________________
                              NaT                          NaN                                NaN                               NaN                            NaN                          NaN                        NaN                           NaN                        NaN                            NaN                             NaN                                NaN            
             01-Jan-2017 00:00:00                       9074.4                            0.85744                            3637.8                        -2919.6                      0.92037                      16256                      0.035938                          0                         -13339                          1125.7                             2415.8            
             01-Jan-2017 01:00:00                       9074.7                            0.85076                            3429.5                        -2920.3                       1.1083                      15964                      0.035288                          0                         -13047                          1076.5                             2381.2            
             01-Jan-2017 02:00:00                       9074.9                            0.84465                              3290                        -2996.5                       1.0244                      15817                      0.026189                          0                         -12824                          1095.5                             2355.1            
             01-Jan-2017 03:00:00                       9077.4                            0.82665                            3053.2                        -2878.1                       0.9999                      15543                      0.019635                          0                         -12668                          1142.3                               2268            
             01-Jan-2017 04:00:00                         9078                            0.80068                            2932.2                        -2855.7                      0.87338                      15481                      0.018089                          0                         -12629                          1158.5                               2311            
             01-Jan-2017 05:00:00                       9079.9                            0.67075                            2787.2                          -2660                      0.69828                      15365                      0.017403                          0                         -12708                          1169.1                             2327.5            
             01-Jan-2017 06:00:00                       9082.6                            0.63896                            2661.7                        -2573.9                      0.00409                      15496                      0.019086                          0                         -12924                          1195.5                             2555.7            
             01-Jan-2017 07:00:00                       9082.7                            0.57101                              2611                        -2382.6                      0.67683                      15607                      0.023495                          0                         -13227                          1184.2                             2727.8            
             01-Jan-2017 08:00:00                       9082.7                            0.55226                            2553.5                        -2077.1                      0.61035                      15663                      0.025813                          0                         -13588                          1165.9                               2860            
             01-Jan-2017 09:00:00                       9084.7                            0.62331                              2392                        -2078.9                      0.10089                      15974                        0.1066                          0                         -13897                          1195.9                             3300.6            
             01-Jan-2017 10:00:00                       9085.5                             1.0216                              2243                        -1612.9                      0.00508                      16105                       0.90292                          0                         -14494                          1212.7                             3561.6            
             01-Jan-2017 11:00:00                       9082.8                            0.96721                            2080.4                          -1197                      0.00387                      16146                        2.4625                          0                         -14951                            1221                             3758.2            
             01-Jan-2017 12:00:00                       9085.3                             1.0963                            1968.2                        -1153.4                      0.00167                      16371                        2.5767                          0                         -15218                          1217.6                             4095.8            
             01-Jan-2017 13:00:00                       9085.1                             1.0842                            2011.4                          -1079                       0.0002                      16513                        1.7942                          0                         -15435                          1232.6                             4181.4            
             01-Jan-2017 14:00:00                       9084.7                            0.97908                            2076.3                        -1023.3                        9e-05                      16764                       0.43023                          0                         -15741                            1246                             4355.6            
If you want to tell readtable that there are 6 header rows, you can do that, and then you'll get a table with 2886-6 = 2880 rows.
T = readtable('n_fot2017-01-12.xls','NumHeaderLines',6)
T = 2880×12 table
            Var1             Var2      Var3       Var4      Var5       Var6      Var7       Var8      Var9    Var10     Var11     Var12 
    ____________________    ______    _______    ______    _______    _______    _____    ________    ____    ______    ______    ______
    01-Jan-2017 00:00:00    9074.4    0.85744    3637.8    -2919.6    0.92037    16256    0.035938     0      -13339    1125.7    2415.8
    01-Jan-2017 01:00:00    9074.7    0.85076    3429.5    -2920.3     1.1083    15964    0.035288     0      -13047    1076.5    2381.2
    01-Jan-2017 02:00:00    9074.9    0.84465      3290    -2996.5     1.0244    15817    0.026189     0      -12824    1095.5    2355.1
    01-Jan-2017 03:00:00    9077.4    0.82665    3053.2    -2878.1     0.9999    15543    0.019635     0      -12668    1142.3      2268
    01-Jan-2017 04:00:00      9078    0.80068    2932.2    -2855.7    0.87338    15481    0.018089     0      -12629    1158.5      2311
    01-Jan-2017 05:00:00    9079.9    0.67075    2787.2      -2660    0.69828    15365    0.017403     0      -12708    1169.1    2327.5
    01-Jan-2017 06:00:00    9082.6    0.63896    2661.7    -2573.9    0.00409    15496    0.019086     0      -12924    1195.5    2555.7
    01-Jan-2017 07:00:00    9082.7    0.57101      2611    -2382.6    0.67683    15607    0.023495     0      -13227    1184.2    2727.8
    01-Jan-2017 08:00:00    9082.7    0.55226    2553.5    -2077.1    0.61035    15663    0.025813     0      -13588    1165.9      2860
    01-Jan-2017 09:00:00    9084.7    0.62331      2392    -2078.9    0.10089    15974      0.1066     0      -13897    1195.9    3300.6
    01-Jan-2017 10:00:00    9085.5     1.0216      2243    -1612.9    0.00508    16105     0.90292     0      -14494    1212.7    3561.6
    01-Jan-2017 11:00:00    9082.8    0.96721    2080.4      -1197    0.00387    16146      2.4625     0      -14951      1221    3758.2
    01-Jan-2017 12:00:00    9085.3     1.0963    1968.2    -1153.4    0.00167    16371      2.5767     0      -15218    1217.6    4095.8
    01-Jan-2017 13:00:00    9085.1     1.0842    2011.4      -1079     0.0002    16513      1.7942     0      -15435    1232.6    4181.4
    01-Jan-2017 14:00:00    9084.7    0.97908    2076.3    -1023.3      9e-05    16764     0.43023     0      -15741      1246    4355.6
    01-Jan-2017 15:00:00    9084.4    0.94576    2106.2    -799.64    0.00044    17276    0.047412     0      -16477    1242.9      4841




