On looking at things, this user is a spammer (ok, allegedly a spammer). Manually checking StopForumSpam found the IP address.
However, there is no indication, to me the administrator/moderator, that this registration attracted the wrath of the Spam Cleaner. The Spam Cleaner log says that it has never been run. The public profile (which can only be gotten to from the Admin Control Panel as the user is not-yet-approved) does have a link to the Spam Cleaner, but the box that pops up doesn't a reason on it. The Admin Control Panel user pages have no indication of the Spam Cleaner at all (or its results).
Checking all the other logs showed nothing.
Am I missing something here, or are we just supposed to guess as to why some users require approval and some don't?