EXPLAIN output shows possible_keys to be NULL and key to be some key

create or replace table t (a int, key(a asc)) engine=innodb;

insert into t select seq * 2 from seq_1_to_100 order by rand(1);

explain select max(200 - a) from t;

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

|  1 | SIMPLE      | t     | index | NULL          | a   |       5 | NULL |  100 | Using index |

select max(200 - a) from t;

max(200 - a)

198

explain select min(200 - a) from t;

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

|  1 | SIMPLE      | t     | index | NULL          | a   |       5 | NULL |  100 | Using index |

select min(200 - a) from t;

min(200 - a)

0