结果对另一个表的总和无效

Question

我有2张桌子

TBL1

INSERT INTO `tbl1` (`id_tbl1`, `name`, `weight`) VALUES
(1, 'data_1', 0.2),
(2, 'data_2', 0.3),
(3, 'data_3', 0.4),
(4, 'data_4', 0.5),
(5, 'data_5', 0.5),
(6, 'data_6', 0.9),
(7, 'data_7', 0.9),
(14, 'data_14', 0.2),
(15, 'data_15', 0.4),
(16, 'data_16', 0.6),
(17, 'data_17', 0.5),
(18, 'data_18', 0.7),
(19, 'data_19', 0.9);

TBL2

INSERT INTO `tbl2` (`id_tbl2`, `id_category`, `id_tbl1`) VALUES
(1, 1, 1),
(2, 1, 2),
(3, 1, 3),
(4, 1, 4),
(5, 1, 5),
(6, 1, 6),
(7, 1, 7),
(14, 3, 14),
(15, 3, 15),
(16, 3, 16),
(17, 3, 17),
(18, 3, 18),
(19, 3, 19);

和我的查询

SELECT SUM(weight)  AS 'total'
FROM tbl2
JOIN tbl1 ON tbl2.id_tbl1 = tbl1.id_tbl1 GROUP BY id_category;

结果应为3.7和3.3，但显示

3.6999999999999997
3.3000000000000003

有人可以告诉我出了什么问题吗？

谢谢。