One answer to the puzzle as why diesel costs more is the fact that under the new ULSD parameters, the diesel fuel produced here in the US now meets world standards for emissions .. and much of it is being exported to a higher profit market!
The "dirty" diesel of years gone by was not marketable anywhere but here.
I would like to see a comparison of cost per gallon to refine diesel vs gasoline.
I know that many localities, (Ca, Co, for example), demand their own special blends, for different seasons and altitudes.