The reason is that the thread or post you see may have been made in the wrong thread so it will have been moved. So if you just post, you may be replying to a thread or posts which were made many hours previously and are no longer there.
I then have to move your posts as well 
One solution I have been doing is locking a thread from which posts have been moved (which generates an error if you press Submit) but then I have to make a note to unlock it, say the following day. That will tend to kill that thread though.
