The reason was this: all company leasning cars should fulfill the standards to be eligible for large tax breaks. All government/state/county/authority/etc. vehicles and so on, should all fulfill the emission standards.
So while it would seem ridiculous that the manufacturers could just take a standard, and "bend" physics to cut consumption in half, it looks like it did. BMW, Audi, VW, Volvo and all the rest are now making full size cars that are making 50-55mpg. Because of regulations. And everyone is pleased to pay less at the pump.
4.3L / 100km is quite a common figure these days, for example http://www.autocar.co.uk/car-review/volvo/v70/first-drives/volvo-v70-1.6d-drive-se
Only God can make random selections.