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