disk space : USED+FREE=TOTAL?

[axt654 at csc.anu.oz]

Hi,
	I've got a DECStation 3100 running Ultrix 2.1. We have a 600Mb hard disk
with three partitions. Why doesn't USED+FREE=TOTAL ? I assume it isn't swap
space as that's a separate partition (isn't it?) so maybe it's because the disk
is fragmented? If so, how do you pack a ultrix disk? It seems we're missing
over 50Mb!

Here is the output from df

Filesyste Total    kbytes   kbytes   %     
node      kbytes   used     free     used  Mounted on
/dev/rz1a   15551    9659    4337    69%   /
/dev/rz1g  271848  215676   28988    88%   /usr
/dev/rz1h  271849   71937  172728    29%   /u


Thnaks in advance for any help
tridge at aerodec.anu.oz.au