Why is this query not hitting an index?

gistfile1.txt

-----------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| activities | CREATE TABLE `activities` (
  `id` int(11) NOT NULL AUTO_INCREMENT,
  `user_id` int(11) DEFAULT NULL,
  `ip` varchar(255) DEFAULT NULL,
  `activity_type_id` int(11) DEFAULT NULL,
  `created_at` datetime DEFAULT NULL,
  `updated_at` datetime DEFAULT NULL,
  `url` varchar(255) DEFAULT NULL,
  `publisher_id` int(11) DEFAULT NULL,
  `local_user_time` datetime DEFAULT NULL,
  `user_badge_id` int(11) DEFAULT NULL,
  `points` int(11) DEFAULT '0',
  `behavior_id` int(11) DEFAULT NULL,
  `event_id` int(11) DEFAULT NULL,
  PRIMARY KEY (`id`),
  KEY `index_activities_on_user_id_and_publisher_id_and_behavior_id` (`user_id`,`publisher_id`,`behavior_id`),
  KEY `index_activities_on_created_at_and_user_id_and_publisher_id` (`created_at`,`user_id`,`publisher_id`)
) ENGINE=InnoDB AUTO_INCREMENT=8412 DEFAULT CHARSET=latin1 | 
+------------+--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
1 row in set (0.00 sec)

mysql> explain SELECT sum(`activities`.points) AS sum_points FROM `activities` WHERE (`activities`.`user_id` = 2688 AND `activities`.`publisher_id` = 2);
+----+-------------+------------+------+--------------------------------------------------------------+--------------------------------------------------------------+---------+-------------+------+-------------+
| id | select_type | table      | type | possible_keys                                                | key                                                          | key_len | ref         | rows | Extra       |
+----+-------------+------------+------+--------------------------------------------------------------+--------------------------------------------------------------+---------+-------------+------+-------------+
|  1 | SIMPLE      | activities | ref  | index_activities_on_user_id_and_publisher_id_and_behavior_id | index_activities_on_user_id_and_publisher_id_and_behavior_id | 10      | const,const |    1 | Using where | 
+----+-------------+------------+------+--------------------------------------------------------------+--------------------------------------------------------------+---------+-------------+------+-------------+
1 row in set (0.00 sec)