Mysql – How does MySQL coerce types during joins

Why do these two queries behave differently:

mysql> explain select FOOBAAR_hhs.relationship_group from FOOBAAR_hhs left join FOOBAAR on FOOBAAR_hhs.relationship_group = FOOBAAR.relationship_group where new_hh_id = 15387929;

| id | select_type | table       | type  | possible_keys | key       | key_len | ref   | rows     | Extra       |
+----+-------------+-------------+-------+---------------+-----------+---------+-------+----------+-------------+
|  1 | SIMPLE      | FOOBAAR_hhs | ref   | new_hh_id     | new_hh_id | 9       | const |       15 | Using where |
|  1 | SIMPLE      | FOOBAAR     | index | NULL          | BAARn     | 126     | NULL  | 83236197 | Using index |

mysql> explain select FOOBAAR_hhs.relationship_group from FOOBAAR_hhs left join FOOBAAR on cast(FOOBAAR_hhs.relationship_group as char) = FOOBAAR.relationship_group where new_hh_id = 15387929;

| id | select_type | table       | type | possible_keys | key       | key_len | ref   | rows | Extra       |
+----+-------------+-------------+------+---------------+-----------+---------+-------+------+-------------+
|  1 | SIMPLE      | FOOBAAR_hhs | ref  | new_hh_id     | new_hh_id | 9       | const |   15 | Using where |
|  1 | SIMPLE      | FOOBAAR     | ref  | BAARn         | BAARn     | 22      | func  |    3 | Using index |

?

The relationship_group members of both relations are typed VARCHAR(20) NOT NULL, but it is only when an explicit CAST is invoked that MySQL permits use of the available index?

(This is MySQL version 5.1.52-log)