He did explain it, just not in that post. The displayed post count does not include deleted posts, or posts from certain sub forums like the sandbox, I think the Lounge/Outside, etc. so the winner of the 15Mth post is not the 15M post that was ever entered in the database.
Exactly... The Lounge counts no posts toward those in the "message" count in the Forum Statistics box. It has (currently) 1,695,419 messages. Now some of those threads used to be part of the counted posts, so not all the 1.6M messages were not counted, but it likely is responsible for much of that 900,000 'gap'. People who post in some of the forums have been here longer than I have and have @ 2000 posts in their profile vs. 20K + in mine, but are also likely have many more actual posts than I have, although since I used to post there, I also have some 'missing'.