Right, and not right again though ... while the board is actually dynamic in building together several infos, it isn't in ALL cases - one of them is the exact user count.
In order to update that one, there's a so called "Update counters" option in our admin CP ... this one gets executed automaticly at a certain time every week (don't ask me which now, seems like it wasn't too long ago) and sets the post count to the exact number, which is in 99,9% of all cases the same or lower as the one displayed before. Reasons for threads to be lower than the actual count are deleted threads, moved threads also mess that up quite a bit, threads which are outdated (we move very, very old threads to another database as this one would slow down waaaaaaaaay too much otherwise) and much more.
Hope this clears things up and enjoy,