I'm not sure the daily fluctuation in oil prices is linked to the longer term costs of fuel manufacture/distribution/retail.
I'm sure there are other members more knowledgeable, but I think retailers will have a contract to buy fuel in advance at an agreed price. I imagine they will also use currency & commodity markets to spread their risks of these daily fluctuations.
I'm not saying they aren't making money, it's just not as straightforward as the headline numbers may seem.