Maybe there is the need to separate real admins (those who actually have an admin job) from people with admin accounts. The latter is not an idiot, the former often is an idiot.
From the Apple Style Guide:
"To maintain the distinction between professional administrators and
Mac OS X
users with administrator accounts, avoid using the noun
itself to describe a person who has an administrator
account in Mac OS X.
Instead, say an admin user or a user with an
account-typically, the owner (or owners) of a particular