Behind the scenes, in what is called the Admin Control Panel, there are a lot of things that can be done to "tweak" the forum, i.e.: clearing caches, updating SQL tables, diagnostics to repair necessary tables, etc. Lots of boring things to you, but a necessity.
Hence, when some of these things are run, it may have recounted posts by users and deducted postst that are no longer counted in some forums (i.e. Test, Voting, etc). It's really hard to say.