Bug in 5.5.30 ?

DROP TABLE IF EXISTS test_et;
CREATE TABLE         test_et (
  exon_id       int unsigned NOT NULL,
  transcript_id int unsigned NOT NULL,
  rank          int          NOT NULL,
  ##
  PRIMARY KEY    (exon_id, transcript_id, rank)#,
  #KEY exon       (exon_id),
  #KEY transcript (         transcript_id)
);

INSERT INTO test_et VALUES
  (289078, 151716, 2),
  (289078, 151717, 2),
  (289078, 151718, 2),
  (289078, 151719, 2),
  (289078, 151720, 2);

#OPTIMIZE TABLE test_et;

SELECT COUNT(*),
       COUNT(DISTINCT exon_id) AS a,
       COUNT(DISTINCT transcript_id) AS b,
       COUNT(DISTINCT rank) AS c
FROM test_et;

## WRONG!
+----------+---+---+---+
| COUNT(*) | a | b | c |
+----------+---+---+---+
|        5 | 5 | 5 | 5 |
+----------+---+---+---+

## MySQL is misusing the index...
SELECT COUNT(*),
       COUNT(DISTINCT exon_id) AS a,
       COUNT(DISTINCT transcript_id) AS b,
       COUNT(DISTINCT rank) AS c
FROM test_et
IGNORE INDEX (PRIMARY);

## RIGHT!
+----------+---+---+---+
| COUNT(*) | a | b | c |
+----------+---+---+---+
|        5 | 1 | 5 | 1 |
+----------+---+---+---+