What you have found in your tests is perfectly correct.
CD/DVD R disks cannot be erased and written to again - that's the real difference. You can always continue adding to an unfilled R disk until it's full (as long as when writing it you don't CLOSE it. If you CLOSE it then it gets converted to a format just like a CD-ROM that cannot be added to).
CD/DVD R/W disks can be used in the same way as R disks - but with the added feature that they can be completely erased and used again. But they DON'T work like a hard drive (or a ZiP drive) - files don't get overwritten when updating, the new version gets written and the old one disappears from view.
Also - confusingly, there are different ways of writing to CD/DVD disks, which may produce different effects - it depends a lot on which software you use for backing up /writing to them.