I am sure it has a lot to do with the refining process and available tank storage space.
This time of year the equipment used to refine diesel is busy making home heating oil. (very similar to #2 diesel and uses much of the same equipment)
Additionally, home heating oil does not need the follow on processing to remove sulfur like ULS Diesel does. That process is very expensive in equipment time and has bumped the price of diesel forever.
Gasoline Blends (I remember reading there are 55 different federal or state mandated blends of gasoline - Are there really 55 different blends of gasoline required in the US? - Democratic Underground
) also complicate the issue and result in large variations of price across the country as blend levels and demand on those blends fluctuate during the year.
For example filling your storage tanks with summer blend and guessing wrong on demand results in not enough storage for the winter blends and a resultant shortage later in the winter if demand stresses the available supply.
No expert; just what I think might explain it.
Lou and Laura with Bella - German Short Hair Pointer
2008 GMC Sierra 2500HD Crewcab SB Allison Duramax
2010 Flagstaff 8526RLWS - Superglide 3300
HAM CALLSIGN - KC3FFW