High mysql sent traffic

we have a wordpress site (Centos6.5+apache2.2.15+mysql5.1.73+php5.3), which are server in vultr, I noticed there has a terrible mysql traffic.

sent data is 6GB per hour. I listed in show processlist; and view the slow query log, it seems no problem.

Here is my my.cnf

port=3306
socket=/var/lib/mysql/mysql.sock
[mysqld]
datadir=/var/lib/mysql
socket=/var/lib/mysql/mysql.sock
user=mysql
symbolic-links=0
skip-external-locking
bind-address=xx.xx.xx.xx
# I need some remote connection, but I have stopped them in crontab, then monitor again, in processlist and slow query log, all the queries have already come from localhost, but the sent traffic still looks terrible.
max_connections=2500
wait_timeout=30
back_log=256
key_buffer_size=256M
table_cache=2048
sort_buffer_size=128k
read_buffer_size=128k
net_buffer_length=128K
read_rnd_buffer_size=3M
myisam_sort_buffer_size=16M
thread_cache=64
query_cache_size=32M
bulk_insert_buffer_size=16M
ft_min_word_len=2

innodb_buffer_pool_size=256M
innodb_additional_mem_pool_size=8M
innodb_log_buffer_size=8M
innodb_flush_log_at_trx_commit=2
innodb_force_recovery=0
innodb_log_file_size=96M

[mysqld_safe]
log-error=/var/log/mysqld.log
pid-file=/var/run/mysqld/mysqld.pid

httpd.conf

Timeout 45
KeepAlive Off
MaxKeepAliveRequests 256
KeepAliveTimeout 3
<IfModule prefork.c>
StartServers 8
MinSpareServers 5
MaxSpareServers 20
ServerLimit 192
MaxClients 192
MaxRequestsPerChild 2500
</IfModule>

what caused such a huge mysql traffic, Is this normal?