Seriously?
No one's threads are being deleted, they aren't going anywhere. The only place they get moved is down into the archives if they're over a yr old, and trust me, they still get counted into your post count. No one is magically going in & deleting your posts or threads--a.) we don't have that kind of time to waste, b.) it would serve no purpose, c.) refer to a & b.
We'll add it to the list of things to look at... But I can flat out tell you that right now we have about 20-30 other things on the list that far supercede the fact that the post counter is fluxing.
Posts aren't being deleted. I think that's the key to this one. Everything else is... well...