Different local taxes would be my guess.
A close examination of gas (oil) revenue shows the ugly truth that it is taxed when the oil is found, taxed again when it's pulled out of the ground, taxed when it's stored, transported, unloaded, stored again, refined, stored, transported, bought by the local station, stored, and finally pumped. Then there's a tax on the "profit" made by the station (a whopping penny a gallon in many cases).
I've probably left out a few of the taxes.
It's a really ugly mess that many are not aware of. Add in the taxes charged on the income that everyone incurs along the way, add that to the price of the gas (the money has to come from somewhere). Hopefully you can see the general drift.