hard drive - Linux different size for df and du for root partition

On my CentOS server, both of the commands are showing different usage for disk / , df shows this output:

Filesystem            Size  Used Avail Use% Mounted on
/dev/xvda2             16G   14G  952M  94% /
/dev/xvda6            996M  258M  687M  28% /tmp
/dev/xvda3             16G  6.5G  8.3G  45% /var
/dev/xvda1            251M   25M  213M  11% /boot

tmpfs                 4.0G     0  4.0G   0% /dev/shm

and du -sh shows

6.9M    /bin/
15M /boot/
123M    /etc
212K    /home/
233M    /lib

27M /lib64/
16K /lost+found/
8.0K    /media/
8.0K    /mnt/
399M    /opt/
959M    /root/
35M /sbin/
8.0K    /srv/
0   /sys/
2.5G    /usr/

4.2G    total

Also tried with NCDU tool, which reports only 5.6GB usage.

How to find hidden disk space?

Answer

See if you have any deleted files still in use

lsof | grep deleted