There are gaps in the number of actual posts vs. the post count on a user's profile due to moved threads, among other things. (Can only remember that moved threads add to the disparity. theymos wrote something about this before)
But the difference is strange "For some, the number looks bigger, vice versa", for example, Jet Cash has 4007 > 3792 while KingScorpio has 3015 < 3047