条件增加后返回的记录数反而多了，为什么呢？

SELECT count(schoolid) AS s1 ,sum(a7) as s2 ,sum(a8) as s3 ,sum(a11) as s4 ,sum(a12) as s5,sum(a16) as s6
FROM [TABLE A]
WHERE a9<>'' and a9 is not null
语句结束后返回1226条


SELECT count([TABLE A].schoolid) AS s1 ,sum(a7) as s2 ,sum(a8) as s3 ,sum(a11) as s4 ,sum(a12) as s5,sum(a16) as s6
FROM [TABLE A],[TABLE B]
WHERE [TABLE B].schooltype like 'D%'
and [TABLE A].a9<>''
and [TABLE A].a9 is not null
and [TABLE B].schoolsys=[TABLE A].schoolid
语句结束后返回1239条

请问多出来13条是怎么回事？