Huge difference between output of "du" and "df"

[root@vrc45sd ~]# mount
/dev/sda3 on / type ext3 (rw)
proc on /proc type proc (rw)
sysfs on /sys type sysfs (rw)
devpts on /dev/pts type devpts (rw,gid=5,mode=620)
/dev/sda1 on /boot type ext3 (rw)
tmpfs on /dev/shm type tmpfs (rw)
none on /proc/sys/fs/binfmt_misc type binfmt_misc (rw)
sunrpc on /var/lib/nfs/rpc_pipefs type rpc_pipefs (rw)
[root@vrc45sd ~]# df -h
Filesystem            Size  Used Avail Use% Mounted on
/dev/sda3              80G   51G   25G  68% /
/dev/sda1              99M   26M   69M  28% /boot
tmpfs                 754M     0  754M   0% /dev/shm
[root@vrc45sd ~]# du -scm /
22466   /
22466   total

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