hmm, when doing a query with a JOIN and a WHERE, why does mysql only use an index for the JOIN part? Why does mysql chose to use the `idxa0` instead of using `idxa2`?

testcase.sql

DROP TABLE article;
DROP TABLE cat;

CREATE TABLE cat (
    id INT AUTO_INCREMENT, 
    weight INT, 
    PRIMARY KEY (`id`)
);
CREATE TABLE article (
    id INT AUTO_INCREMENT, 
    cat_id INT, 
    score INT,
    PRIMARY KEY (`id`)
);
ALTER TABLE article ADD INDEX idxa0 (cat_id);
ALTER TABLE article ADD INDEX idxa1 (cat_id, score);
ALTER TABLE article ADD INDEX idxa2 (score, cat_id);

INSERT INTO cat (id, weight) VALUES 
(1, 6),
(2, 3);

INSERT INTO article (cat_id, score) VALUES 
(1, 1), (1, 1), (1, 1), (1, 1), (1, 1), (1, 1), (1, 1), (1, 1), (1, 1), (1, 1), (1, 1), (1, 1), (1, 1), (1, 1), (1, 1), (1, 1), (1, 1), (1, 1), (1, 1), (1, 1), (1, 1), (1, 1), (1, 1), (1, 1), (1, 1), (1, 1), (1, 1), (1, 1), (1, 1), (1, 1), (1, 1), (1, 1), (1, 1), (1, 1), (1, 1), (1, 1), (1, 1), (1, 1), (1, 1), (1, 1), (1, 1), 
(2, 1), (2, 1), (2, 1), (2, 1), (2, 1), (2, 1), (2, 1), (2, 1), (2, 1), (2, 1), (2, 1), (2, 1), (2, 1), (2, 1), (2, 1), (2, 1), (2, 1), (2, 1), (2, 1), (2, 1), (2, 1), (2, 1), (2, 1), (2, 1), (2, 1), (2, 1), (2, 1), (2, 1), (2, 1), (2, 1), (2, 1), (2, 1), (2, 1), (2, 1), (2, 1), (2, 1), (2, 1), (2, 1), (2, 1), (2, 1), (2, 1)
;

EXPLAIN 
SELECT article.*
FROM article
WHERE cat_id = 1 AND score > 5;


EXPLAIN 
SELECT article.*, cat.weight
FROM article, cat
WHERE article.cat_id = cat.id AND score > 5;