查询结果和表的结果不一样啊？

SELECT 商品品种进货统计.品种, 商品品种进货统计.进货量, 商品品种销售统计.销售量, [进货量]-[销售量] AS 库存量
FROM (SELECT 品种, Sum(进货数量) as 进货量 FROM 进货表 GROUP BY 品种) AS 商品品种进货统计 
LEFT JOIN (SELECT 品种, nz(Sum(销售数量),0) AS 销售量 FROM 销售明细表 GROUP BY 品种) AS 商品品种销售统计 ON 商品品种进货统计.品种=商品品种销售统计.品种;