问为什么对于同一查询，MySQL会显示不同数量的Rows_examined？

EN

# Time: 130615 11:35:59
# User@Host: root[root] @ localhost [127.0.0.1]
# Query_time: 32.198606  Lock_time: 0.000000 Rows_sent: 0  Rows_examined: 68851
SET timestamp=1371314159;
SELECT * FROM `projectaddresses` WHERE `ID_PROJECT` = 77 ORDER BY `ID_PROJECTADDRESS` ASC LIMIT 25 OFFSET 0;
# Time: 130615 11:37:22
# User@Host: root[root] @ localhost [127.0.0.1]
# Query_time: 30.030193  Lock_time: 0.000000 Rows_sent: 0  Rows_examined: 163319
SET timestamp=1371314242;
SELECT * FROM `projectaddresses` WHERE `ID_PROJECT` = 77 ORDER BY `ID_PROJECTADDRESS` ASC LIMIT 25 OFFSET 0;