MySQL version 5.5.14

According to the document, by the default, slave does not log to its binary log any updates that are received from a master server.

Here are my config. on the slave:

# egrep 'bin|slave' /etc/my.cnf
relay-log=mysqld-relay-bin
log-bin = /var/log/mysql/mysql-bin
binlog-format=MIXED
sync_binlog = 1
log-bin-trust-function-creators = 1

mysql> show global variables like 'log_slave%';
+-------------------+-------+
| Variable_name     | Value |
+-------------------+-------+
| log_slave_updates | OFF   |
+-------------------+-------+
1 row in set (0.01 sec)

mysql> select @@log_slave_updates;
+---------------------+
| @@log_slave_updates |
+---------------------+
|                   0 |
+---------------------+
1 row in set (0.00 sec)

but slave still logs the updates that are received from a master to its binary logs, let's see the file size:

-rw-rw---- 1 mysql mysql  37M Apr  1 01:00 /var/log/mysql/mysql-bin.001256
-rw-rw---- 1 mysql mysql  25M Apr  2 01:00 /var/log/mysql/mysql-bin.001257
-rw-rw---- 1 mysql mysql  46M Apr  3 01:00 /var/log/mysql/mysql-bin.001258
-rw-rw---- 1 mysql mysql 115M Apr  4 01:00 /var/log/mysql/mysql-bin.001259
-rw-rw---- 1 mysql mysql 105M Apr  4 18:54 /var/log/mysql/mysql-bin.001260

and the sample query when reading these binary files with mysqlbinlog utility:

#120404 19:08:57 server id 3  end_log_pos 110324763     Query   thread_id=382435        exec_time=0     error_code=0
SET TIMESTAMP=1333541337/*!*/;
INSERT INTO norep_SplitValues VALUES ( NAME_CONST('cur_string',_utf8'118212' COLLATE 'utf8_general_ci'))
/*!*/;
# at 110324763

Did I miss something?