This is a recurrent problem...
This is most likely caused by direct access to the table with db-tools, improper handling of database entries, for example: through custom programs (that might set values to this field directly in database level); it's also possible that the values have been set in debugging mode and were not adjusted afterwards by the author of this modification.
For further information, visit note 1887820 - Incorrect User Lock Status (UFLAG) in table USR02.