Any ideas what would cause the admin CP to report the wrong number of users awaiting approval?

It is cached for a bit of time, I think it's 15 minutes, so if you've been to the CP and came back, and new users have registered during that time, that could be one reason.

This happened over an 18 hour period.

However, I had some crashed tables and three registrations had no details (username, display name etc.)

I've deleted the oddities and rebuilt the tables and it looks like it's ok now.

...except that I haven't had one registration today and I'm getting paranoid