michaelxu@michaelxu-server:~$ df -h
Filesystem Size Used Avail Use% Mounted on
/dev/sda1 71G 65G 2.0G 98% /
none 495M 224K 495M 1% /dev
none 501M 104K 501M 1% /dev/shm
none 501M 376K 501M 1% /var/run
none 501M 0 501M 0% /var/lock
How come the used + available size does not equal the disk size for /dev/sda1?