Transaction IDs not the same in same transaction?

So I was finally able to get a compiled binary for the code in this
thread(thanks Magnus):
http://archives.postgresql.org/pgsql-general/2005-06/msg00709.php

So everything seemed to be fine with my GetCurrentTransactionID()
function call returning the txn ID for each query I would run(as far
as I could tell). Then I tried running a txn with multiple queries,
and instead of just having one txn ID, each query had it's own. Does
that make any sense? I was under the impression that a regular
transaction block would have one txn ID assigned to it for its
duration.

Here's the query I ran:

BEGIN;
UPDATE partners SET
partner_name = 'partner #5',
activity_status_id = 1
WHERE partner_id = 5;

UPDATE partners SET
partner_name = 'partner #7'
WHERE partner_id = 3;

COMMIT;

Should that have had the same txn ID for both of those? The
GetCurrentTransactionID call occurs in a trigger, would that have an
impact on the ID?

Thanks,
Steve