Compressing CD image files



Why is it when I try to compress a CD image file, even with the maximum compression (and most processor intensive) option, I only get something like a 0.9 compression ratio?

If I manually copy out the files of an (unprotected) CD using Nero or even Windows Explorer, and compress it, I would get much better compression ratios than doing it from image files. Presumably, this is partly because image files add additional information to it. However, from the compression ratios, is it correct to deduce that image files scramble up the original information as well, in an almost random fashion (since purely random content can not be compressed?)? Between the two ways an image file can be written, scrambled up information and information appended to the end of unadulterated data, it seems the former method would be predominant. Why is this?

Does the end result of compression of image files depend much on the original content of the CD? Is there a way to compress such files with decent compression ratios?