timestamp with timezone and time zone name

Hello,

I am wondering, why following two values result in a shift by 3.5 hours. I
would expect them to be identical.

I understand that canonical time zone names could be ambiguous at times but I
think IST is not one of them.

Any explanation?

---------------
test=# select '2012-08-07 05:24:56.758891+05:30'::timestamptz;
timestamptz
----------------------------------
2012-08-07 05:24:56.758891+05:30
(1 row)

test=# select '2012-08-07 05:24:56.758891 IST'::timestamptz;
timestamptz
----------------------------------
2012-08-07 08:54:56.758891+05:30
(1 row)

shridhar(at)bheem ~$ psql test
psql (9.1.4)
Type "help" for help.

test=# select version();
version
---------------------------------------------------------------------------------------------------------
PostgreSQL 9.1.4 on x86_64-unknown-linux-gnu, compiled by gcc (GCC) 4.7.0
20120505 (prerelease), 64-bit
(1 row)
---------------

TIA.
--
Regards
Shridhar