Since I can't see what exactly is within these tables, I'll take a shot in the dark:
This could be due to the data-types within the tables. If you're trying to vertically concatenate character array table entries, for instance, if the two tables do not have the same length for these arrays, it would have the same effect as attempting the below:
If their concatenation is necessary, it may be necessary to convert these char arrays to strings or cells of char arrays. For some information regarding this, I'll direct you here.
If this is not the case, could you please supply us with more information regarding what is in the tables? E.g. run the below: