That would be because the version on the CD is older than the version from the net. The only way you could do it (not really recommended but..) is to comment out all the internet sources, run
Code:
sudo apt-get update
sudo apt-get install ubuntu-desktop
then re-enable the sources and update. I can't say for sure that this will though because of dependences, it may just be simpler to install from the net. If you don't have a connection on the system and you can use another connection you can use
Code:
apt-get --print-uris install ubuntu-desktop
to get apt to print the URIs to download and where to save them so you can put them on a CD and transfer them to the system.