[CentOS] df command reports inaccurate results?

Can anyone help make sense of this?  This is an ext3 partition.  It's
only showing 403GB out of 426GB used, but then it says only 632MB
available?  Where'd the extra ~25GB go?

[scarolan at server]$ df -H /disks/vrac5

Filesystem             Size   Used  Avail Use% Mounted on
/dev/sdb2             426GB  403GB  632MB 100% /disks/vrac5