Controlling Reuslts with Limit

Hi,
 I was reading through Bruce's on line . I found follwing bit unclear...

"Notice that each query uses ORDER BY . Although this clause is not required,
LIMIT without ORDER BY returns random rows from the query, which would be
useless. "

When I run a query several time  I get the same results as given
flipr=# select song_id from songs  limit 5;
 song_id
---------
     945
     946
     947
     948
     949
(5 rows)

flipr=# select song_id from songs  limit 5;
 song_id
---------
     945
     946
     947
     948
     949
(5 rows)

flipr=# select song_id from songs  limit 5;
 song_id
---------
     945
     946
     947
     948
     949
(5 rows)

flipr=# select song_id from songs  limit 5;
 song_id
---------
     945
     946
     947
     948
     949
(5 rows)

flipr=# select song_id from songs  limit 5;
 song_id
---------
     945
     946
     947
     948
     949
(5 rows)

flipr=# select song_id from songs  limit 5;
 song_id
---------
     945
     946
     947
     948
     949
 I just want to know what exatly --"LIMIT without ORDER BY returns random rows
from the query" --means
Regards