BUG #7924: Trigger update function don't take correct values

The following bug has been logged on the website:

Bug reference: 7924
Logged by: Maria
Email address: mv(dot)gonzalez(at)cir(dot)es
PostgreSQL version: Unsupported/Unknown
Operating system: RedHat
Description:

Hi!

I have a trigger that when you update a value on a table, makes an insert
into another table.

The trigger looks like the following:

BEGIN
set datestyle to 'sql';
set datestyle to 'european';
IF (TG_OP = 'UPDATE' and (OLD.situation!=NEW.situation)) THEN
INSERT INTO table_B VALUES (OLD.id,NEW.situation,NOW());
END IF;
RETURN NULL;
END;

For exemple, If I do the next querys:
--> update table_A set situation='Started' where id='23';
On table_A appears:
id | situation
--------+-----------
23 | Started

On table_B appears:
id | situation | when
--------+-----------+---------------------
23 | Started | 2013-03-08 12:33:35

--> update table_A set situation='Waiting' where id='23';
On table_A appears:
id | situation
--------+-----------
23 | Waiting

On table_B appears:
id | situation | when
--------+-----------+---------------------
23 | Started | 2013-03-08 12:33:35
23 | Started | 2013-03-08 12:33:37

I don't understand why after the second update, the insert done on table_B
is incorrect, and has the same values that the first one.

I've been looking for some kind of explanation like cache or something, but
I couldn't find anything.

Thanks in advance.

Maria