Perhaps yoou failed to notice that the S400 has a Tiddly (oh you got that) electric motor and the S500 has a motor three times as powerful yes 60HP not 20HP(No way near similar!) , meaning, guess what? more power, faster acceleration but also and crucially three times the Regenerative Braking! meaning, guess what? More Power, in saved energy! Hence, More Miles Per Gallon!. But hey lets not stop there... The S500 has the Battery Pack in the Boot, why was that? well I recall reading that its bigger, Meaning, guess what? More Power! are you spotting a trend yet? Dont forget that the MPG is a combined calculation based on more short (electric) journies and a few longer (petrol) journies. so by improving the short journey ability and improving recovery the MPG figure becomes a lot greater. and the emissions a lot less!
Basically this car can achieve the figures stated by being good on shorter (all electric) journies! having a huge 60HP Boost instead of 20HP and recovering so much more regenerativly.
More electrical power and better recovery = More efficient use of petrol!!