strange row count?

COuld someone explain me why a query:
mlotdev=> SELECT min(date_part('epoch',t_max-now())) FROM aibuf WHERE flag='A';
min
-----

(1 row)

showed one row (empty!!!!) while the query (equivalent one):
mlotdev=> SELECT date_part('epoch',t_max-now()) FROM aibuf WHERE flag='A' order by 1;
date_part
-----------
(0 rows)

showed 0 ?

This happens both in psql an libpq interface and is sth strage to me (but
maybe there is some point :)

The query/table details are non important ( You should get the same results on
other examples).

My postgresql is 7.0.3

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