Why are 2 rows affected in my `INSERT ... ON DUPLICATE KEY UPDATE`?

I'm doing an INSERT ... ON DUPLICATE KEY UPDATE for a PRIMARY KEY in the following table:

DESCRIBE users_interests;

+------------+---------------------------------+------+-----+---------+-------+
| Field      | Type                            | Null | Key | Default | Extra |
+------------+---------------------------------+------+-----+---------+-------+
| uid        | int(11)                         | NO   | PRI | NULL    |       |
| iid        | int(11)                         | NO   | PRI | NULL    |       |
| preference | enum('like','dislike','ignore') | YES  |     | NULL    |       |
+------------+---------------------------------+------+-----+---------+-------+

However, even though these values should be unique, I'm seeing 2 rows affected.

INSERT INTO users_interests (uid, iid, preference) VALUES (2, 2, 'like')
ON DUPLICATE KEY UPDATE preference='like';

Query OK, 2 rows affected (0.04 sec)

Why is this happening?

EDIT

For comparison, see this query:

UPDATE users_interests SET preference='like' WHERE uid=2 AND iid=2;

Query OK, 1 row affected (0.44 sec)
Rows matched: 1  Changed: 1  Warnings: 0

From the manual:

> With ON DUPLICATE KEY UPDATE, the > affected-rows value per row is 1 if > the row is inserted as a new row and 2 > if an existing row is updated.

So you know whether you updated a row (duplicate key) or just inserted one: http://dev.mysql.com/doc/refman/5.0/en/insert-on-duplicate.html