[TriLUG] Puzzling df output

[Randy Barlow]

So I've got a volume that's been filling up lately, and I removed some 
stuff from it and was puzzled by the output of df:

booty rdiffbackup # df -h --sync
Filesystem            Size  Used Avail Use% Mounted on
...
blah, blah, blah
...
/dev/mapper/backupvg-rdiffbackup
                       459G  441G     0 100% /data/rdiffbackup
...

How can I have used 18G less than the size, but still have 0 bytes 
available?