My guess is that your posts are being deemed to be spam by one of the forum moderators and they are just removing them to clean up the forum. I do not know of any other reason for post counts to be reduced. Try to make sure your posts offer some value to others in the forum.
Something unusual is going on. Lots of posts are being deleted around the forum, but people aren't being notified by PM that their posts are being deleted.
It would be nice to know what posts/threads are being removed.