different results when using value of optimizer_use_condition_selectivity=4 and =1

SET @@old_mode=zero_date_time_cast;

CREATE TABLE t1 (a TIME);

INSERT INTO t1 VALUES ('0000-00-00 10:20:30'),('0000-00-00 10:20:31');

INSERT INTO t1 VALUES ('0000-00-01 10:20:30'),('0000-00-01 10:20:31');

INSERT INTO t1 VALUES ('31 10:20:30'),('32 10:20:30'),('33 10:20:30'),('34 10:20:30');

SET @@optimizer_use_condition_selectivity=1;

SELECT * FROM t1 WHERE a='0000-00-01 10:20:30';

SELECT * FROM t1 WHERE a='0000-00-01 10:20:30' AND LENGTH(a)=8;

EXPLAIN EXTENDED SELECT * FROM t1 WHERE a='0000-00-01 10:20:30' AND LENGTH(a)=8;

SET @@optimizer_use_condition_selectivity=4;

SELECT * FROM t1 WHERE a='0000-00-01 10:20:30';

SELECT * FROM t1 WHERE a='0000-00-01 10:20:30' AND LENGTH(a)=8;

EXPLAIN EXTENDED SELECT * FROM t1 WHERE a='0000-00-01 10:20:30' AND LENGTH(a)=8;

MariaDB [test]> SET @@optimizer_use_condition_selectivity=1;

Query OK, 0 rows affected (0.00 sec)

MariaDB [test]> SELECT * FROM t1 WHERE a='0000-00-01 10:20:30' ;

+----------+

| a        |

+----------+

| 34:20:30 |

+----------+

1 row in set (0.00 sec)

MariaDB [test]> SET @@optimizer_use_condition_selectivity=4;

Query OK, 0 rows affected (0.00 sec)

MariaDB [test]> SELECT * FROM t1 WHERE a='0000-00-01 10:20:30';

Empty set (0.00 sec)