Oh, sorry, I meant to post about this since we usually get a LOT of questions when it happens.
Whenever posts or thread are deleted the post counts don't always immediately drop. Every once in a while we do some site optimizations and this includes "syncing up" the actual post count of members.
We've been doing some early Spring Cleaning of old threads that don't (by themselves) add anything by staying on the forum, like the "count high before staff post" threads. If you participate a lot in those threads and we remove them, you may see a significant post count drop.