The Null Value -- different results in Sql syntaxes

Question

I have 2 different queries (with different purposes) working perfectly but there is something I can not grasp yet about the Null value

Here is one query:

 select NCLI , LOCALITE , CAT 
 from CLIENT
 where CAT not in (’B1’,’C1’)
 or LOCALITE not in (’Lille’,’Namur’);

It shows exactly what I want Including the rows where CAT has a null value. Now this seems perfectly logical to me.

The only thing that disturbs me is that the query below only worked when I specified I did wanted the null rows in CAT to show up

select NCLI , LOCALITE , CAT from CLIENT
where CAT <> ’C1’
or CAT is null
or LOCALITE = ’Namur’;

When I don't add -- or CAT is null -- the null values don't show up. Could someone tell me what I'm not grasping here yet? Thanks.