Yup-just got off the phone with Honda in Torrance.
Apparently this measurement method was pushed by Toyota, America.
The formula is City mileage (not highway) and its improvement in the weight/size class overall.
Because the Civic is a compact, it is compared to all compacts only.
The Accord gets compared to the mid-sized.
The Ford Escape to other SUV's.
Ditto the Highlander.
More info here:
http://aceee.org/transportation/hybtaxcred.htm