Explain output difference in Extra column

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

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

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

|    1 | PRIMARY     | alias1 | ALL  | NULL          | NULL | NULL    | NULL | 14   |                                    |

|    1 | PRIMARY     | t2     | ALL  | NULL          | NULL | NULL    | NULL | 20   | Using join buffer (flat, BNL join) |

|    3 | SUBQUERY    | t3     | ALL  | NULL          | NULL | NULL    | NULL | 20   | Using where                        |

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

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

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

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

|    1 | PRIMARY     | alias1 | ALL  | NULL          | NULL | NULL    | NULL | 14   | Using where                        |

|    1 | PRIMARY     | t2     | ALL  | NULL          | NULL | NULL    | NULL | 20   | Using join buffer (flat, BNL join) |

|    3 | SUBQUERY    | t3     | ALL  | NULL          | NULL | NULL    | NULL | 20   | Using where                        |

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

set optimizer_record_context=ON;

CREATE TABLE t1 (a INT) ENGINE=MyISAM;

INSERT INTO t1 VALUES (7),(0),(9),(3),(4),(2),(5),(7),(0),(9),(3),(4),(2),(5);		

CREATE TABLE t2 (b INT, c INT) ENGINE=MyISAM;

INSERT INTO t2 VALUES 

(0,4),(8,6),(1,3),(8,5),(9,3),(24,246),(6,2),(1,9),(6,3),(2,8),

(4,1),(8,8),(4,8),(4,5),(7,7),(4,5),(1,1),(9,6),(4,2),(8,9);

create table t3 like t2;

insert into t3 select * from t2;

explain 

SELECT max(a+b+c) FROM t1 AS alias1, ( SELECT * FROM t2 ) AS alias 

WHERE EXISTS ( SELECT * FROM t3 WHERE b = c )  OR a <= 10;

SELECT context INTO DUMPFILE 'context1.txt' FROM INFORMATION_SCHEMA.OPTIMIZER_CONTEXT;

source context1.txt