df and du disagree, or "How to get 1.1GB of data onto a 650MB CD"

[re2823@Safe-mail.net]

Hi all.

I have a strange problem: I have an iso image file which
(according to ls) is 569M in size:

poota:~# ls -lh image.iso
-rw-r--r-- 1 root root 569M 2006-09-12 15:12 image.iso

I also have an empty directory:
poota:~# ls -l temp/
total 0

Then I mount the iso, loopback style:
mount image.iso temp/ -o loop

Now for the problem...
The relevant line from "df -h":
poota:~# df -h
Filesystem            Size  Used Avail Use% Mounted on
/root/image.iso       569M  569M     0 100% /root/temp

And the result from running "du -sh":
poota:~# du -sh temp/
1.1G    temp/

Just to really baffle me, I've burnt this iso image onto a 650MB
CD-ROM, mounted it, and it too has 1.1GB of data on it:

poota:~# du -sh /media/cdrom0
1.1G    /media/cdrom0

But the "df -h" output seems more reasonable:
server:~# df -h
Filesystem            Size  Used Avail Use% Mounted on
/dev/hdd              569M  569M     0 100% /media/cdrom0

Could anyone please tell me how there can be such a huge
difference between the output from df and du?

My real problem comes when I want to remove one small file from
the file system, and turn it back into an iso - the resulting iso
image is 1.1GB (and I can no longer write it to a CD!)

Thanks, Jaime