another thing to consider, is that the US emission standards for diesel are ridiculously tighter than europe, i.e. US diesel nox is 0.07 grams per mile vs 0.29 for europe (I *think* Cali targets 0.02, and ignores the effects that reduced hydrocarbons have).
Diesel Cars in Europe vs. America - Why Diesel Vehicles Are Expensive in US
nox on a diesel requires extra fuel to deal with.
And a road test vs a dyno can have a great effect on peak nox. climbing a hill into a headwind in first is gonna make a lot of emissions per mile.
I'm not really sure what is going on behind the scenes here at volkswagen OR the epa.
It does seem to me that the US has been somewhat hostile towards the small diesel market, and it would be naive to think it is about tailpipe crap from a handful of cars (that also happen to get awesome mileage).