i think it's due to the query; do a select * from sysusers in any database. there's no Builtin\Administrators in that list.
the query is not showing implicit users who have access regardless of whether you added them or not; those users like sa get access because they are sysadmins, they do not need to be added to each db's sysusers list. It's just getting explicitly granted users and their permissions.
Lowell