no cond returns diff res than cond or !cond

Good day,

feline=> select version();                           version                            
---------------------------------------------------------------PostgreSQL 7.0.2 on i686-pc-linux-gnu, compiled by gcc
2.95.2
(1 row)

feline=> select count(*) from produkte;count 
-------36405
(1 row)

feline=> select count(*) from produkte where artnummer != NULL or
artnummer = NULL;count 
-------36347
(1 row)

cenes_test=> select version();                          version                           
-------------------------------------------------------------PostgreSQL 7.1.3 on i686-pc-linux-gnu, compiled by GCC
2.96
(1 row)

cenes_test=> select count(*) from produkte;count 
-------36384
(1 row)

cenes_test=> select count(*) from produkte where artnummer = NULL or
artnummer != NULL;count 
-------36326
(1 row)

2nd query 58 less than first.

bug?

Markus Bertheau