I think that the figures have something to do with the way that the official consumption figures combine urban and extra-urban consumption. For example, the ads for the new hybrid version of the four-door Porsche Panamera claim an mpg of 91.1 and emissions of 71g/km (along with a combined output of 416hp and a 0-62 figure of 5.5 seconds).
But if you look at the small print, the extra-urban mpg is just 38.2 mpg and the car will only travel 21 miles on battery alone (but has an official urban mpg of 0). So if you only use your Porsche for short journeys in town, you might achieve the claimed mpg, but I don't imagine many people would buy a Porsche for that.