I confirm such behavior. It’s by design. URE does not apply any restrictions to a user with ‘administrator’ role for the single site WordPress installation as such user is a superadmin there.
I agree that checkboxes available for the selection at the “Other Roles” dialog window for the ‘administrator’ role may confuse a user. I will fix this with the next update.
You can change this logic under WordPress multisite, as it’s not enough to have ‘administrator’ role in order to be superadmin there. It’s possible via custom filter ure_not_block_other_roles_for_local_admin.
