explicit joins wrong planning

Hi
SELECT version();
PostgreSQL 7.3.4 on i386-pc-linux-gnu, compiled by GCC i386-linux-gcc
(GCC) 3.3.2 20031005 (Debian prerelease)

Let's say I have 3 tables:

groups (
groupid integer primary key,
name varchar,
begindate date
);

offsets (
offset_id integer,
groupid integer references groups,
offset_value integer
);

events (
offset_id integer references offsets,
event_date date,
primary key (offset_id,event_date)
);

explain analyze select *
from
groups g
join offsets o using (groupid)
join events e on (e.offsetid=o.offset_id and
e.event_date=g.begindate+o.offset_value)
where g.name='some_name';

Postgres doesn't use join on these both fields and doesn't use index
scan properly.
I get:
Hash Cond: ("outer".offset_id = "inner".offset_id)
Join Filter: ("outer".event_date = ("inner".begindate +
"inner".offset_value))

Why?
I lost few hours trying to fix it and I found, that copying one of these
conditions into where clause solved my problem:

explain analyze select *
from
groups g
join offsets o using (groupid)
join events e on (e.offsetid=o.offset_id and
e.event_date=g.begindate+o.offset_value)
where g.name='some_name' and e.offsetid=o.offset_id;

Join Filter: ("outer".event_date = ("inner".begindate +
"inner".offset_value))
Nested Loop...
Join Filter: ("outer".offset_id = "inner".offset_id)

Why? What was I doing wrong?

Regards,
Tomasz Myrta