df -k vs. du -s

[Don O'Neil]

My /var partition is showing a different value for a df -k on the file
system vs a du -s on the file system:

df -k
Filesystem    1K-blocks      Used     Avail Capacity  Mounted on
/dev/ad0s1a     2026030    889544    974404    48%    /
devfs                 1         1         0   100%    /dev
/dev/ad0s1e     2026030       964   1862984     0%    /tmp
/dev/ad0s1d     8122126   1997988   5474368    27%    /usr
/dev/ad0s1f     8122126   5301938   2170418    71%    /var

du -s /var
993261  /var

Any ideas why I would see this? Where is the other 4+G? Do I have a bunch of
bad sectors in the file system or is it majorly fragmented? If so, how do I
find out what the problem is?

The other partitions match between the df and du... I'm running 6.1 if that
makes a difference.