it might be the motorway thing
i reckon that the lower pwoered cars will do less motorway miles as they are going to be more "driveable" whereas the bigger powered cars will be set up for big power top end and so they'll sacrifice a bit of fuelin bottom end

i think there might also be a fueling issue if you have all 8 injectors opening up when you plant it as opposed to just 4 for normal driving

or i might be talking bollocks