I don't believe Ford made any real changes between the 2016 and 2017 C-Max to improve fuel economy. I believe the only difference for the C-Max is that the EPA changed their test procedure. While most cars are getting lower fuel economy numbers, a few (such as the C-Max) are testing better. I believe, in the C-Max's case, it may test slightly better using the new procedure combined with Ford being overly cautious with the EPA numbers after downgrading the C-Max's fuel economy twice (from the initial 47/47/47) when it came out in 2013.
Seems to me I recall reading that the "coast down" methodology to determine the road load coefficients which are used to setup the dynamometer was changed from a coast down speed of 70 mph to maybe 50? mph. I'll see if I can find EPA changes in the quidelines and why. This change might increase / decrease the relationship of the road load coefficients associated with the road load components.
But EPA has adjusted FE of prior years models also as shown in djc first post. I doubt the manufacturers re-tested prior year models. So, I question whether one can determine if 2017 models that were actually tested are really more / less efficient than prior models absent retesting all prior year models. The 2017 Fusion Hybrid was tested. AFAIK, there is no requirement for manufacturers to test the same model every year like the C-Max.
Remember these EPA numbers are to be used for comparison of vehicles among makes and models. I wouldn't bet the house that a 2017 FFH gets better highway FE than a 2016 FFH all other things being equal. But, it is a feather in Ford's hat that the EPA Hwy rating of the FFH has increased.
I wouldn't bet the house that a 2017 FFH gets better highway FE than a 2016 FFH all other things being equal.