Was he a new user? His last seen time would have been at login.
It only pulls the timestamp from the U_Laston field. If their not a registered user it wont see them. Only .threads scripts update this field though.
I'm starting to think that this box isn't quite right since it isn't using something that was ment to track users every step of the way. The core .threads script, ubbt.inc.php would need to be modified so that every call to send_header would update a field used to track real "last seen" times for each user.