Yeah, I did that too. Fast user switch isn't the same as logging out.
I actually discovered a work around/bug that isn't the correct way. I don't know if it's been fixed but if you add a new user and I think go to the name box to change the name, you can press delete twice and remove the new user and the other logged in user.
The correct way is to switch to the other user, log out and go to the admin account and delete.