Indexes on that table also cause the table to change the last ddl time, which you might missed to point.

SQL> select to_char(last_ddl_time,'hh24-mi-ss'),object_name from user_objects where object_name='N';

TO_CHAR( OBJECT_NAME
-------- ----------------------------------------
11-31-46 N

SQL> create index x_ind on N(no);

Index created.

SQL> select to_char(last_ddl_time,'hh24-mi-ss'),object_name from user_objects where object_name='N';

TO_CHAR( OBJECT_NAME
-------- ----------------------------------------
11-32-36 N