well it really depends on how you drive as well. I have had only one car in the last twenty or so years where I could not consistently beat the EPA ratings. Plus throw in that most ares have ethanol formulations and that will lower your mileage.

Far too many race from light to light, keep on the gas even when going downhill and over the limit, brake late and even run with tires under inflated.

anecdotal, my current car a s28 Z4 uses a four cylinder turbo. 23/34 I think are the official ratings, I have yet to average a tank under 30 and did 40 on pure highway at speeds over 70. Granted its a manual which lets me abuse the rev range a bit more than the 8 speed auto offered but its simply down to learning to drive your car within its best performance parameters. I still have fun, run it up fast, but mostly just drive relaxed.

Does it mean that some cars are not performing to what companies claim. I am quite sure they aren't. I could not get a 2013 Mustang V6 above its EPA AVG to save my life.