Then there are three other options (are you sure the CUA is not active?):
1) An exit is active in SU01 which modifies the screen if the field is not initial.
2) Someone modified the screen variant in SHDB and the standard SU01 now has a custom screen.
3) Someone modified SU01.
Which SAP release are you on? (that will influence which exits are effective).
Cheers,
Julius