Actually, I addressed the comments noted in your post, but not directly as a reply to yours. Someone else pointed out other symptoms of the same thing.
Here's the answer.
If the last post in a thread is deleted, the total post count will be one more than the number of posts that are "available". The reason being that the deleted post still occupies a spot in the thread. It's just hidden from view. If the last 3 are deleted, the total post count will be 3 greater than the number of visible posts, etc.
It's happening right now because we keep getting hit by spammers and we're roasting everything they write. In some cases, this involves every single thread on the site.
I expect it to be much less of a problem once I'm able to programmatically address the issue of spam.
Bob