This user has given two answers, but does not show up in our user list here. I first waited a bit, since I thought maybe it just takes time for that page to update. However after several hours of waiting, I noticed 2 new users show in the list (who signed up later than the aforementioned user who was "last seen 7 hours ago", which is longer ago than the creation times of the 2 new users).
I've kept track of the new users quite closely on this site, and never once saw this before, so it seems it's an isolated incident. Is there an explanation of what might have happened?