Maybe There is a Problem about auto_increment_increment

[five small]

I'm Very Sorry To interrupt~~Code First,Maybe it's a BUG...

create table test(id int not null primary key auto_increment unique);
Query OK, 0 rows affected (0.04 sec)

set @@auto_increment_offset=10;
set @@auto_increment_increment=3;

insert into test values(null),(null),(null),(null),(null);

select * from test;
+----+
| id |
+----+
| 3 |
| 6 |
| 9 |
| 10 |
| 13 |
+----+

I Think There must be Wrong about number 9 and number 10, And What's Worse ,SomeTimes The 3rd number is 10 like following,I Swear I use THe Same Santences,But I get different results.
+----+
| id |
+----+
| 3 |
| 6 |
| 10 |
| 13 |
| 16 |
+----+