I had similar problems with Modern Warfare 2 back in the days.
Steam wanted me to download 14GB even though I had both discs in my tray already.
Try deleting everything Steam has downloaded so far and try to install it again.
that would completely defeat the purpose of me trying to save time by installing the game from the disc. if i did that, i would be spending even more time reinstalling every single game i ever got.
lol sry I meant you should delete all Shogun 2 files that Steam already downloaded on your HDD.