Sounds like smalls read my link. Or maybe he wrote it!
From that article........
Gasoline and diesel fuel prices have traditionally followed set seasonal patterns from year to year. In spring and summer, the peak driving season, gasoline sells at a premium to diesel fuel. In the autumn, demand for distillate fuels (heating oil and diesel) picks up ahead of the winter at the same time that gasoline demand begins to soften. Refineries begin to build inventories of high sulfur distillate fuel (heating oil) late in the summer, while diesel fuel consumption increases in the fall due to farm use and trucking of goods ahead of the holidays. Heating oil prices put a floor under diesel prices through the winter, since if diesel were selling at a discount to heating oil, diesel could be used for home heating.
Sounds like heating oil sets the price for diesel in winter. Heating oil is always at it's highest price in the middle of the winter, when people need it the most.
Its all about the demand, and then who needs it the most.