BUG #3607: timestamp is behaving strangely

The following bug has been logged online:

Bug reference:      3607
Logged by:          Cade Cairns
Email address:      cadec@otii.com
PostgreSQL version: 8.1.9
Operating system:   Mac OS X 10.4.10
Description:        timestamp is behaving strangely
Details:

test=> select '2007-05-11 09:33:01.167885'::timestamp without time zone,
('epoch'::timestamp without time zone + '1178875981.167885
seconds'::interval)::timestamp without time zone;
         timestamp          |         timestamp
----------------------------+----------------------------
 2007-05-11 09:33:01.167885 | 2007-05-11 09:33:01.167885
(1 row)

test=> select '2007-05-11 09:33:01.167885'::timestamp without time zone =
('epoch'::timestamp without time zone + '1178875981.167885
seconds'::interval)::timestamp without time zone;
 ?column?
----------
 f
(1 row)

Why?