Frage zu explain

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60 rows in set (0.00 sec) liefert die q. Explain jedoch behauptet, daß nur 42 rows gelesen wurden.

60 rows in set (0.00 sec)

mysql> explain  select ent from forum where att = 'thread' ;
+----+-------------+-------+------+---------------+------+---------+-------+------+-------------+
| id | select_type | table | type | possible_keys | key  | key_len | ref   | rows | Extra       |
+----+-------------+-------+------+---------------+------+---------+-------+------+-------------+
|  1 | SIMPLE      | forum | ref  | att           | att  | 130     | const |   42 | Using where |
+----+-------------+-------+------+---------------+------+---------+-------+------+-------------+

explain?