My best guess is that you've had to deal with batches of spammers from time to time, and deleted their accounts.
(rightfully so, too) But, this is probably a bad guess because spammers usually post spam...The Rocks and Diamonds forum (which I'm also a member of) has a visual confirmation for new members where they have a series of pictures and have to click on the ones that match a certain criteria. Go to the registration screen there http://www.artsoft.org/forum/ and you'll see what I mean.
[EDIT] Actually, after seeing the member list, I think not that my guess is so bad. There are multiple senseless, indecipherable usernames, and their URL's are all going to cosmo sites, webhosting spots, and other junk like that. I still think that they would post random crap, though. Maybe they're slackers, failing at their spamming lives.
(Oh, and by the way, I don't like spammers)
