Incorrect result on aggregate function(distinct)+group by

[FlyingMao]

Prepare data：

mysql> CREATE TABLE sbtest1(`id` int(10) unsigned NOT NULL,`k` int(10) unsigned NOT NULL DEFAULT '0',`c` char(120),`pad` int(11) NOT NULL,PRIMARY KEY (`id`),UNIQUE KEY (`k`));
d) values(19,19,19);
Query OK, 0 rows affected (0.05 sec)

mysql> insert into sbtest1 values(1,1,'id1',1),(2,2,'id2',2),(3,3,'id3',3),(4,4,'id4',4),(5,5,'id5',1),(6,6,'id6',2),(7,7,'id7',3),(8,8,'$id8$',4),(9,9,'test',3),(10,10,'中',3),(11,11,'i_',4),(12,12,'_g',5),(13,13,'y_u',6),(14,14,'20%',14),(15,15,'a_1',15),(16,16,16,-1),(0,0,0,0),(17,17,'new*\n*line',17),(18,18,'a',18);
Query OK, 19 rows affected (0.01 sec)
Records: 5  Duplicates: 0  Warnings: 0

mysql> insert into sbtest1(id,k,pad) values(19,19,19);
Query OK, 1 row affected (0.01 sec)

dble result:

mysql> select count(distinct id),pad from sbtest1 group by pad;
+----------------------------+-----+
| COUNT(DISTINCT sbtest1.id) | pad |
+----------------------------+-----+
|                          1 |  -1 |
|                          1 |   0 |
|                          1 |   1 |
|                          1 |   2 |
|                          1 |   3 |
|                          1 |   4 |
|                          1 |   5 |
|                          1 |   6 |
|                          1 |  14 |
|                          1 |  15 |
|                          1 |  17 |
|                          1 |  18 |
|                          1 |  19 |
+----------------------------+-----+
13 rows in set (0.01 sec)

mysql result:

mysql> select count(distinct id),pad from sbtest1 group by pad;
+--------------------+-----+
| count(distinct id) | pad |
+--------------------+-----+
|                  1 |  -1 |
|                  1 |   0 |
|                  2 |   1 |
|                  2 |   2 |
|                  4 |   3 |
|                  3 |   4 |
|                  1 |   5 |
|                  1 |   6 |
|                  1 |  14 |
|                  1 |  15 |
|                  1 |  17 |
|                  1 |  18 |
|                  1 |  19 |
+--------------------+-----+
13 rows in set (0.00 sec)