It's possible that the engine power / automatic transmission shift profile / maximum RPM / etc. is being manipulated to output less emissions at the cost of acceleration, top speed, ride comfort, etc.
The most fuel-efficient / eco ride is rather "boring" by the standards of car enthusiasts and reviewers and I guess the cars wouldn't be configured for that mode by default.
Or, you just pay more for a car that has those performance characteristics both in taxes/registration fees and in fuel.
That said, cheating like this is really bad on a very large scale, and should probably face fines and maybe criminal charges, unfortunately discovery is likely to fall outside U.S. juristiction so it would be hard to prove enough to bring criminal charges against the appropriate managers and executives.
The most fuel-efficient / eco ride is rather "boring" by the standards of car enthusiasts and reviewers and I guess the cars wouldn't be configured for that mode by default.
(Just a guess though.)