isn't it because of the fact that a mb is not actually 1000kb, but 1024? So a CD manufacturer says that you have 700mb=700*1000 kb=700,000kb, but the file system actually only allows you to use 700,000/1024=683mb. That's the way it works on Wintel - is it the same on Mac?
Still doesn't get you down to 660mb, though I think the previous poster was right - cd's need some system files on them which are saved when the contents are actually burned.