Why do I have to check if a value is NULL?

I have the following query:

CREATE OR REPLACE VIEW muni_no_one_point_inside AS
SELECT 
      m.gid
    , scripts.stx_extract(m.geom, 2)
FROM muni m
LEFT JOIN LATERAL (
    SELECT ST_Collect(x.geom) as geom
    FROM (
        SELECT geom
        FROM munitextosmal mtm
        WHERE mtm.geom && m.geom
        AND ST_Relate(m.geom, mtm.geom, 'T********')
    )x
) as xx ON true
WHERE st_numGeometries(
    st_intersection(m.geom, xx.geom)
) <> 1

That is, I want to obtain the geometries that either do not intersect with any point (NULL) or that intersect with more than one point .

With the previous query, the NULL values ​​are not taken, I have to force it to take the NULL values, that is, in the WHERE clause I must check that the number of geometries is NULL or different from 1.

WHERE st_numGeometries(
    st_intersection(m.geom, xx.geom)
)  IS NULL OR st_numGeometries(
    st_intersection(m.geom, xx.geom)
) <> 1

My question is, why isn't it enough to check that it is different from 1?

How does PostgreSQL internally decide that NULL is not different from 1?