I tried it out and got a similar result. But when I execute a:
select extend_id, file_id, blocks from dba_extents where segment_name='MY_TABLE_NAME' and owner='MYSELF'
I got again 6 extents.

maybe it's something like a small bug. If you have a look a the sql of the views dba_extents and dba_tables you may find out why you got different results.