Why does SteamGift think i own a game when in fact i only own the standalone dlc.

why does steam and steamgift keep thinking i own shogun 2 when i own shogun 2 fall of the samurai. this is getting annoying since i can enter any give away for shogun 2 since it think i own it. this also happen on the steam store not allowing me to own the game on my account with out making a new steam account and buying on that account. from what i notice it seem to think it the same game ( kind of is the same game) since i can enter give away for the dlc for shogun 2 fine as it think i own it. why not just have steam/steam gift recognize them as different games so you wont have the problem.