explain for update shows using filesort, but it's not used

CREATE TABLE `t1` (

  `id` int(10) unsigned NOT NULL auto_increment,

  `t` date,

  PRIMARY KEY (`id`),

  KEY `t` (`t`)

);

INSERT INTO t1 (t) values ('2013-11-14'), ('2013-11-15'),('2013-11-16'),('2013-11-17'),('2013-11-18'),('2013-11-19'),('2013-11-20');

EXPLAIN UPDATE t1 SET t = CURDATE() WHERE id = (SELECT * FROM (SELECT id FROM t1 ORDER BY t ASC LIMIT 1) tmp) LIMIT 1;

FLUSH STATUS;

UPDATE t1 SET t = CURDATE() WHERE id = (SELECT * FROM (SELECT id FROM t1 ORDER BY t ASC LIMIT 1) tmp) LIMIT 1;

SHOW STATUS LIKE 'sort%';

id      select_type     table   type    possible_keys   key     key_len ref     rows    Extra

1       PRIMARY t1      ALL     NULL    NULL    NULL    NULL    7       Using where

2       SUBQUERY        <derived3>      ALL     NULL    NULL    NULL    NULL    2

3       DERIVED t1      ALL     NULL    NULL    NULL    NULL    7       Using filesort

SHOW STATUS LIKE 'sort%';

Variable_name   Value

Sort_merge_passes       0

Sort_range      0

Sort_rows       0

Sort_scan       0