Where did you see that? Seems odd that the rating on the car has actually gone up this year but it is charged the tax. I do know that the rating system has some bizarre stuff that goes on - the rating on the window sticker is not always the rating that is used for CAFE purposes.
BTW, if you want to see how screwed up the government is in this regard, do a little studying of why so many big SUV's and pickup trucks have those "FlexFuels" emblems on them. You'd be surprised what their true CAFE rating is.
GONE: 2012 BMW 550 M-Sport
2016 Audi A7 Prestige
2016 Lexus GX 460---Wife's car
2015 BMW 1600 GT---2016 BMW S1000R---2012 Ducati Panigale