Surprising NULL behavior

SELECT CASE WHEN NULL NOT IN ('A', 'B')       THEN 1 ELSE 0 END AS a FROM DUAL;
SELECT CASE WHEN NULL     IN ('A', 'B')       THEN 1 ELSE 0 END AS a FROM DUAL;
SELECT CASE WHEN NULL NOT IN ('A', 'B', NULL) THEN 1 ELSE 0 END AS a FROM DUAL;
SELECT CASE WHEN NULL     IN ('A', 'B', NULL) THEN 1 ELSE 0 END AS a FROM DUAL;