Likely the difference can be explained away somewhat by the difference in testing. The EPA testing is much more stringent, the same exact configuration showing

Likely the difference can be explained away somewhat by the difference in testing. The EPA testing is much more stringent, the same exact configuration showing much better mileage in European testing than US testing. The difference between US and Japan are even worse. The rest of the difference is simply vehicle preference, the US having more sales in the truck category.