Another reason might be that the person who decides on the end user price (ie. the airfield owner) doesn't follow market prices on a day by day basis, but simply takes the price of the last bowser of 100LL he received, divides that by however liters it contained, adds a standard uplift to make a tiny profit, and charges you that price until the next bowser arrives.
So you might still be paying prices that were set two weeks ago, depending on the volume of sales. Obviously when the price is going up steadily, this works out to your advantage. But when the fuel price is going down steadily (like in the last few days) this gives you a disadvantage.
If you want to save some money, best idea is to shop around a bit for the lowest fuel price, and split the difference between that price and your home base between the owner and the pilot.
Or, as others have suggested, convert to mogas or Jet-A.