Ahh, I think I see what is going on. If a user account has contributed say a story, then the user URL is aliases:
http://de.castlecops.com/user/paul
If a user has not contributed, then the alias is not created:
http://de.castlecops.com/user/15
You can compare...
http://de.castlecops.com/
"By mrrockford at Thu, 03/11/2005 - 05:55 "
"mrrockford" is aliases. If you check "Who's new", none at the moment are aliased.
So there seems to be a condition (without checking the code).