Does limit impact affected rows or not? I have this table:

CREATE TABLE IF NOT EXISTS `usergroups` (
      `id` int(11) unsigned NOT NULL AUTO_INCREMENT,
      `user_id` int(11) unsigned NOT NULL,
      `group_id` smallint(5) unsigned NOT NULL,
      PRIMARY KEY (`group_id`,`user_id`),
      KEY `id` (`id`),
      KEY `user_id` (`user_id`)
    ) ENGINE=InnoDB  DEFAULT CHARSET=utf8 COLLATE=utf8_unicode_ci AUTO_INCREMENT=5496 ;


when run this query:

EXPLAIN SELECT `UserGroup`.`user_id` FROM   `usergroups` AS `UserGroup` WHERE  `UserGroup`.`group_id` = 1 limit 30


its out put is:

1       SIMPLE        UserGroup      ref  PRIMARY  PRIMARY  2        const  543      Using index


I think something is wrong! because it affected 543 rows but I think it should affect 30 rows at maximum. Is it true?
Limit does have an impact on how many rows are selected/affected. 543 is just the number of rows which match the joins and the where clause.