problem about mixed binlog-format in percona5.5

i checked my binary log and find the unsafe SQL still log use SBS
but my log format is mixed,why?

#120405 9:15:46 server id 5551 end_log_pos 3817 Query thread_id=81681 exec_time=25 error_code=0
SET TIMESTAMP=1333588546/!/;
INSERT INTO toocle_news_base.news_cate1 SET nid = ‘6630489’,category = ‘4511’, post_time = now()

mysql> show variables like ‘binlog_format’;
| Variable_name | Value |
| binlog_format | MIXED |
1 row in set (0.00 sec)

I’m not really sure what you are asking but binlog_format=MIXED actually means that most of the logging is SBR.

Ref:The manual binary-log-mixed

SET TIMESTAMP=1333588546/!/; ensures that the NOW() function returns the same value on slaves