Not complete synchronization (it's possible?) Why?

[kRel]

This is my first msg: greetings to all. . .

Using FFS for some years and I am satisfied, but there are some things I do not understand because of my poor knowledge of english. For the same reason, it is possible that the subject has already been treated and I have not been able to locate it and find the answer. I apologize for this....

I always wondered how come every time, right after making a new sync, if you make a new comparison, are found numerous items still not synchronized.

Sometimes, to get a clean sheet comparison, you must do the synchronization process also 3/4 times. Why ?