The mileage difference between 4WD/AWD system and the FWD system is really more about the additional loss of energy to friciton in the drive train.

My '05 FEH still has only around 18000 miles on it. I checked the engine oil and air filter and found both were a lot dirtier than I expected (it's been getting oil changes more frequently than required). Just changing an air filter can make a huge noticeable difference in fuel economy (I've heard "claims" that it can be as much as 20% -- I find this hard to believe though I think that 10% is easily believable) and this in itself can account for a 3 mpg fuel economy difference.

The point is you have to be careful when comparing a car that's been service for a while to one that's brand new. I expect that even if Ford didn't change a thing between model years and theoretical economy would be expected to be unchanged, you'd still notice a difference just because of things like comparing the mileage of a car with a dirty air filter to a car with a clean one.