Can login as "admin" despite no "admin" user account

Despite there being no user account with the name “Admin” I am able to login as this user.

I have a WPMU site running two sites, and I have double checked both sites, there is definitely no user account with this name.

I just examined the WP_Users table in the database and sure enough I can see an entry there with the user_login “Admin”.

Why would this not have been removed from the database when the user was deleted from the list of users via WP admin?