Time zone specifications, abbreviations vs full names

Either I am exceptionally confused, or the documentation in 8.5.3 appears to be wrong.  Could someone clarify what I'm
missing?

The documentation states: "In short, this is the difference between abbreviations and full names: abbreviations always
representa fixed offset from UTC, whereas most of the full names imply a local daylight-savings time rule, and so have
twopossible UTC offsets."  "PST" is given as a specific example of an abbreviation in this case.  But it appears that
using"PST" gives you a time zone rule: 

CREATE TABLE t (
   z TIMESTAMP WITH TIME ZONE
);

insert into t values ( '2011-03-13 01:00 PST'::timestamptz);
insert into t values ( '2011-03-13 01:00-7'::timestamptz );
update t set z = z + '2 hours'::interval;

select * from t;
           z
------------------------
 2011-03-13 04:00:00-07
 2011-03-13 03:00:00-07
(2 rows)

--

But if "PST" really did imply a fixed offset, shouldn't the results be the same?

--
-- Christophe Pettus
   xof@thebuild.com