Why does the Scion xD have such a poor green rating?

The Scion xD ranks 56th in the green ratings at 54, while getting 33/27 mpg. However, the Forester and Outback get much poorer mileage (29/22 and 28/23 respectively), and therefore would have much more emissions. Given that the amount of emissions for each vehicle would be amplified when you take into consideration the emissions over the life of the vehicle, yada yada yada. Why is the xD ranked so far below vehicles that it has CONSIDERABLY better mileage than?