That's a bit weird. Some threads have been moved into Forum Games, and posts made before being moved count toward official post counts, but I don't know if that explains the discrepancy.

Forum Games is the only public forum that doesn't count toward post counts. Some or all (the Restricted Forums category has this property, but some of the subforums do not—I'm not sure if it's inherited or not) of the private forums do not count.

One thing I could do is recount all the posts. This would update official post counts to their proper values, taking into account current omission settings.