This administrative user is created during install by the person/developer settings up the system. This is a admin username/password that always has access to the system. The admin login credentials are set in the following file:
admin/control/access.php
Ideally this is used only to set up the system or to access the system if a store owner has forgot their password. Once you have created staff via the staff management screen, you can do one of the following:
1 Comment out the 2 constants in the 'admin/control/access.php' file.
2 Remove the 'admin/control/access.php'.
To regain access via this admin staff member, replace the deleted file or uncomment out the constants, depending on your preference.