What you actually ordered was the turbo 'look' rims for your C2S and that is what you got. The wheels that are stock for the 997tt are of a different offset than the ones that you received on your narrow body Carrera. If you check carefully you will discover that the 'turbo' wheels provided on the 997tt are not the same as those installed on the narrow body cars. As such, they will come with a different tire. As John, above, mentions, only the Carrera Sport XRR rims come with 305s in back for the slender cars.

Dan