Yeah, vBulletin stores a static number of posts in each thread. It knows how many posts to give you per page, so it's the ceil(post count / posts per page). Pretty straightforward.

I haven't poked around the vBulletin files to say for sure, but I'm guessing that when a post is added, deleted, etc., vBulletin doesn't fully retally the number of posts in a thread -- it just increments/decrements the existing stored number. Apparently the way it does this isn't all that reliable, at least under certain scenarios. I wouldn't think it'd be that "expensive" to do a quick retally when there's a significant action in a thread, but maybe that is a concern for extremely busy forums. Having the option for a mod/admin to manually trigger a retally for a single thread seems like it shouldn't be a big deal to put in place. As far as I can remember, this version of vBulletin only has an option to retally the posts for every thread...nothing to target a specific discussion.