Maybe, but I suspect the real reason is that they have not learned how to drive for mileage.

My proof is my own first full tank on Inverse MidLife Crisis. The car's first tank was calculated at 61 mpg at the pump, the display showed 57.9. In fairness the 61 figure is very likely to be way off due to an overfill. The display is also high by about 3% so the actual corrected figure is something like 56 mpg for a car rated at 46/51.

IMLC is still on factory oil and the temps have been in the single digits on some nights (some days are in the 50-60ies and rainy) where I drive the car and the car still always gets well over the EPA numbers. I attribute the good figures to skill, having a 2nd shift job with light traffic, and having a 66% highway trip to work that is not too short. I personally do not think the "break in" period has anything to do with it.

I actually hope you are right. If you are IMLC is going to be doing really great when she gets broke in.