カラムの最大値？ - MAX()関数の謎

さて、何故でしょうか？

mysql> select hoge_id from gao;
+---------+
| hoge_id |
+---------+
| 1       |
| 2       |
| 3       |
| 4       |
| 5       |
| 6       |
| 7       |
| 7       |
| 9       |
| 10      |
| 11      |
| 12      |
| 13      |
| 1       |
| 15      |
| 16      |
| 17      |
| 18      |
| 19      |
| 20      |
| 21      |
| 22      |
| 23      |
| 24      |
| 25      |
| 26      |
| 27      |
| 28      |
| 29      |
| 30      |
| 31      |
| 32      |
| 33      |
| 34      |
| 35      |
| 36      |
| 10      |
+---------+
37 rows in set (0.00 sec)

mysql> select max(hoge_id) from gao;
+--------------+
| max(hoge_id) |
+--------------+
| 9            |
+--------------+
1 row in set (0.00 sec)