Not sure how IPB works, but with vBulletin it counts 100% accurate your posts. But whenever u disable a section's post count, it stops counting posts from that instant only. Lets say offtopic counts posts and u have 1000 posts. I disable post count on that section and u will remain with 1000 posts and only the next posts won't count.
But, there's also a nice feature called Post Recount. That will reset ALL post counts for all users and recount from zero, considering only the sections that have post count enabled. That's what we did and will do again til we get all the sections that should have post count disabled/enabled.
I'm pretty sure that either IPB didn't work this way or we are disabling post count in sections that weren't disabled in IPB. Or just IPB is a piece of shit that never learned to count properly.
So, expect more post count drops/increases in the next days.