NOT IN vs. NOTEXISTS

Question

为什么第一个查询返回0行但第二个查询返回行？

SELECT *
FROM   dbo.GW_ASO_SCRUBBED
WHERE  ACCOUNT_NUMBER NOT IN (SELECT Acct_num
                              FROM   dbo.ACCOUNT_X_MASTER)

SELECT *
FROM   dbo.GW_ASO_SCRUBBED s
WHERE  NOT EXISTS (SELECT Acct_num
                   FROM   dbo.ACCOUNT_X_MASTER M
                   WHERE  s.ACCOUNT_NUMBER = M.ACCT_NUM)