dbms_space.space_usage gives different result to 'row_enabled' tables at same table_owner, why?

 Hi:
I'm looking for a way to 'shrink' table size so that no need to bother developers doing archiving job, so that more space will be released, and HOPEFULLY, it will sustain us in the next year of 2010!

2 candidate tables:

SQL> select table_name ,  row_movement from dba_tables where owner='PROD' and row_movement ='ENABLED';

TABLE_NAME                     ROW_MOVE
------------------------------ --------
ADDRESS                        ENABLED
USERACCOUNT                    ENABLED


I execute the package in this way -

begin
   dbms_space.space_usage(
      segment_owner      => 'PROD',
      segment_name       => 'ADDRESS',
      segment_type       => 'TABLE',
...
end;
/

Address -
FS1 Blocks = 54 Bytes = 221184
FS2 Blocks = 87 Bytes = 356352
FS3 Blocks = 108 Bytes = 442368