timestamp weirdness

We are having some strange behavior with PostgreSQL 7.1.3 where rows 
are inserted with a timestamp that is out of order... here is the 
table description and a few rows:


                             Table "event_page"
   Attribute   |           Type           |             Modifier
--------------+--------------------------+----------------------------------
  user_id      | oid                      |
  session_id   | character varying(24)    |
  course_ident | character varying(128)   |
  page         | character varying(32)    |
  date         | timestamp with time zone | default "timestamp"('now'::text)


  69714 |   61381 | yVLsSYYpZIa1G54VnDY7qcz2 | COG_001_ELEARNING | 
15.html | 2002-01-31 18:07:21+08
  69715 |   61381 | yVLsSYYpZIa1G54VnDY7qcz2 | COG_001_ELEARNING | 
16.html | 2002-01-31 18:07:40+08
  69717 |   61453 | uZuNbGXsWnXXRECH6PGeFdtf | COG_001_ELEARNING | 
1.html  | 2002-01-31 18:08:14+08
  69718 |   61453 | uZuNbGXsWnXXRECH6PGeFdtf | COG_001_ELEARNING | 
15.html | 2002-01-31 18:09:30+08
  69719 |   61453 | uZuNbGXsWnXXRECH6PGeFdtf | COG_001_ELEARNING | 
16.html | 2002-01-31 18:07:49+08



The oid correctly reflects the order of the insertion of the rows... 
but look at the timestamp - the last row has a timestamp _2 minutes 
before_ the previous row. How could this be happening? We know row 
69719 was inserted _after_ 69718, by probably about 30 seconds.

Thanks much.

Elaine Lindelef