Data logging problem (Data acquried are not consistent with time)

I tried to get data with xPC target. and I saved data into workspace. Theoretically, If I set sampling time as 0.001 and run it for 1sec, 1000 data should be acquired. However, I got only 995 or around there. To figure out the problem, I saved data with time and it shows that data do not start to be logged at 0sec. For example, data start to be logged at 0.005 sec. So 0.005s to 1s, 995 data.... Can someone help me with this problem???

[Rajiv Ghosh-Roy]

How are you logging the data?