Because Cadillac has the rest of GM to make cars. CAFE standards are on the whole fleet. So if Chevy has a subcompact with a turbo that gets 40mpg, that brings the whole fleet's fuel efficiency ratio up. They will also sell more of those than the big v8 cadillacs, and if I remember correctly, the CAFE requirements are weighted by sales. Similar story for the other auto manufacturers with huge, diverse portfolios.