Partly that, and the USA's much higher car dependence in general. I'd wager the average adult in the USA spends more time in a petrol station than the average European.
Edit: Bad googling, sorry. 57 mpg was proposed for new cars.
US gallons happen to be smaller too, fwiw
NB: the above mpg numbers are computed using US gallons (3.785 litres)
I meant, for anyone that missed it, that commuting by bus/train/tram/metro is much more common in urban areas in Europe than in the USA.