The case of the missing 24GB

[Bruco]

So if I run a df -h on my home server a particular line jumps out at me:

# df -h
Filesystem       Size    Used   Avail Capacity  Mounted on
/dev/ufs/root    496M     46M    410M    10%    /
devfs            1.0K    1.0K      0B   100%    /dev
/dev/ufs/tmp     496M     76K    456M     0%    /tmp
/dev/ufs/usr     285G    235G     26G    90%    /usr
/dev/ufs/var     1.7G    112M    1.4G     7%    /var

Er, last I checked taking 235 from 285 leaves you with 50 - not 26. I know some rounding goes on here, but 24GB appears to be "missing"! It looks like in the cases of almost all the lines there is some hard disk space that is not accounted for, but in the case of /usr it's certainly the most noticeable.

/usr is mounted on the same logical drive that everything else is - which is a RAID-5 drive provided by three 160GB drives on a Highpoint controller.

Any ideas? Is there something I just don't get here? Are the measurements showing me two different things based on a cluster size (similar to the size vs. size on disk in Windows)?

Thanks.