My disk free space in ubuntu has been stolen

This is output after i run df -h

Filesystem      Size  Used Avail Use% Mounted on
/dev/vda1        30G   19G  9.1G  68% /
none            4.0K     0  4.0K   0% /sys/fs/cgroup
udev            487M  4.0K  487M   1% /dev
tmpfs           100M  348K  100M   1% /run
none            5.0M     0  5.0M   0% /run/lock
none            498M     0  498M   0% /run/shm
none            100M     0  100M   0% /run/user
/dev/sda         25G  6.4G   17G  28% /mnt/volume-sgp1-01

I want to identify what are 19G being used.

So i run below command

du -h --max-depth=10 --one-file-system / | sort -rh | head -25

to get following output

14G /
12G /var
8.8G    /var/lib
8.6G    /var/lib/mysql
8.1G    /var/lib/mysql/getapple_phpbb
1.6G    /var/www
916M    /usr
546M    /var/www/tskthai.com/public_html
546M    /var/www/tskthai.com
517M    /var/www/tskthai.com/public_html/wp-content
464M    /lib
431M    /var/www/ereaderok.com/public_html
431M    /var/www/ereaderok.com
396M    /home
381M    /var/www/himaparn.com/public_html
381M    /var/www/himaparn.com
378M    /var/lib/mysql/getapple_paysmile
368M    /lib/modules
306M    /var/www/tskthai.com/public_html/wp-content/uploads
295M    /usr/lib
255M    /var/www/getapple.net/public_html
255M    /var/www/getapple.net
250M    /opt
244M    /usr/share
242M    /opt/datadog-agent

you can see that the main directory being used is / which is only 14gb

So where are the remaining 5GB are being used?

Please advise.