Two queries faster than than one?

I have a table with columns:

CREATE TABLE aggregates (
    a VARHCAR,
    b VARCHAR,
    c VARCHAR,
    metric INT
    KEY test (a, b, c, metric)
);

If I do a query like:

SELECT b, c, SUM(metric) metric
FROM aggregates
WHERE a IN ('a', 'couple', 'of', 'values')
GROUP BY b, c
ORDER BY b, c

The query takes 10 seconds, explain is:

+----+-------------+------------+-------+---------------+------+---------+------+--------+-----------------------------------------------------------+
| id | select_type | table      | type  | possible_keys | key  | key_len | ref  | rows   | Extra                                                     |
+----+-------------+------------+-------+---------------+------+---------+------+--------+-----------------------------------------------------------+
|  1 | SIMPLE      | aggregates | range | test          | test | 767     | NULL | 582383 | Using where; Using index; Using temporary; Using filesort |
+----+-------------+------------+-------+---------------+------+---------+------+--------+-----------------------------------------------------------+

If I also group by/order by column a, so it doesn't need temporary/filesort, but then do the same thing in another query myself:

SELECT b, c, SUM(metric) metric
FROM (
    SELECT a, b, c, SUM(metric) metric
    FROM aggregates
    WHERE a IN ('a', 'couple', 'of', 'values')
    GROUP BY a, b, c
    ORDER BY a, b, c
) t
GROUP BY b, c
ORDER BY b, c

The query takes 1 second and the explain is:

+----+-------------+------------+-------+---------------+------+---------+------+--------+---------------------------------+
| id | select_type | table      | type  | possible_keys | key  | key_len | ref  | rows   | Extra                           |
+----+-------------+------------+-------+---------------+------+---------+------+--------+---------------------------