Probably a refinement in the test procedure/estimate by the EPA... My best guess.
They're trying to make it more relevant to what is seen in "real-world" driving. And some of the small-displacement turbocharged engines have a bad habit of overstating what they're capable of (*cough* ECOBOOST *cough*). The 1.4T didn't seem to fail to deliver when we took the '14 2LT to NC and back - especially for how much **** we had in the car.My 2012 ECO MT also saw the "official" rating drop from 42 to 40 MPG highway. Seems to me like the EPA is generally tightening up on the ratings.