When you create new sites and are using "Related Sites Mode", it creates these admin users just in case you ever change it back to not using Related Sites Mode, otherwise if you did that, there would be no admin user. You can safely delete those users from the db if you want to, they won't come back on their own but each time you create a new site it will create a new one for that site, so if you don't want it there in the database you would have to delete it again after each new site is created.
Hope it helps,
Joe