MySQL not using multiple column index for some queries

MySQL not using multiple column index for some queries

I have a table with several million records on MySQL in an MyISAM table.
Very simplified, it's like this:
CREATE TABLE `test` (
`text` varchar(5) DEFAULT NULL,
`number` int(5) DEFAULT NULL,
KEY `number` (`number`) USING BTREE,
KEY `text_number` (`text`,`number`) USING BTREE
) ENGINE=MyISAM DEFAULT CHARSET=latin1;
It's populated with this data:
INSERT INTO `test` VALUES ('abcd', '1');
INSERT INTO `test` VALUES ('abcd', '2');
INSERT INTO `test` VALUES ('abcd', '3');
INSERT INTO `test` VALUES ('abcd', '4');
INSERT INTO `test` VALUES ('bbbb', '1');
INSERT INTO `test` VALUES ('bbbb', '2');
INSERT INTO `test` VALUES ('bbbb', '3');
When I run the following query:
EXPLAIN SELECT * FROM `test` WHERE (`text` = 'bbbb' AND `number` = 2)
It returns 'number' as the key to use. But the following query:
EXPLAIN SELECT * FROM `test` WHERE (`text` = 'bbbb' AND `number` = 1)
Returns 'text_number' as key to use, which would make more sense to me as
this combined key matches exactly with the 2 columns in the WHERE. On
these amount of records the performance isn't an issue, but on several
million records the query which uses the 'text' index takes 4 seconds, and
the one that uses 'text_number' index is finished in several milliseconds.
Is there a logical explaination for this? How can I change the index that
MySQL uses the index? I know I can use USE INDEX but I want MySQL to be
able to find the best plan to execute the query. This is on MySQL 5.1 and
5.5, same results.