Re: One surprise...
Partly because of the higher calorific value of heavy oil (opposed to petroleum by mass) partly due to the fact that in order to burn 'cleanly' diesel engines run at a higher temperature...
...except higher combustion temperatures has an undesirable side-effect of creating higher concentrations of oxides of nitrogen, though oxides of carbon are reduced.
The EPA have always been interested in NOx, Europe were mistakenly focussed on CO2 until recently.
VW have 'cheated' their performance figures by circumventing US laws, which gives the advantage of making their engines look more efficient, when in reality this comes about through unfair means. Plenty of other manufacturers could make engines at least as efficient if they didn't play by the rules either.
Oh, and yeah my Jag is rather efficient too but let's not let that cloud the aura of German bullshit that is all pervasive in the car industry.