Missing hard drive space. df and du can't agree

[tsan]

Hi I have been searching for the 'lost' hard drive space on the root partition of one of my servers.
6.2-RELEASE-p5 FreeBSD 6.2-RELEASE-p5
When I peform a df -h for the root partition I get this
gol-goroth# df -h
Filesystem Size Used Avail Capacity Mounted on
/dev/ad0s1a 496M 453M 3.2M 99% /

Using the du -xh / | grep M command I get the output of

1.0M ./etc/rc.d
4.2M ./etc
19M ./boot/kernel
19M ./boot
3.5M ./lib
4.0M ./sbin
33M .

This would indicate that the / partition has 33mb of data on it. What I don't understand is why there is a difference between the 2 commands. When I try and copy data onto the / partition I get an error that it is out of space. I have only included the du output of M as the complete listing results in 33Mb being used.

Is there anything I have potentially missed ?

Any suggestions would be appreciated.