I think the post difference is due to the "trash being emptied" when Xper came in to fix that spam issue. I'm not sure how the "online at once record" works with the forum internals. Anyways, I think those records really don't mean anything anymore. They used to be some cool stat, but most forums' records now show a period of spamming or some DoS attack. For example, the online at once record at reboot.pro is from one of those types of attacks.