With the much more sophisticated engine controls and better cylinder head design engines can run much leaner at light throttle steady state conditions which accounts for the higher hiway mileage cars get these days. However, richer mixtures are required for acceleration and you can't get away from that so the mileage has improved as much as the hiway mileage.
The thing I find amazing about new cars is the amount of power with great economy they get.
For example: I purchased a 85 Daytona Turbo new in 85 it was arguably the most powerful FWD compact car of the day. It made 150hp/150ft-lbs and got 25-27 mpg around town and 32-34 mpg on the hiway. My wife's 12 Focus SEL makes 160hp/148ft-lbs and gets 32-34 mpg in the same around town driving and gets 38-39 mpg on the hiway. Then take my 13 ST it gets the same mileage as the Daytona and makes 100ho/100ft-lbs more than the Daytona.