Confused. That's just a cash flow issue.The stuff that's already in the tank has to be sold at a price that will pay for the new stuff coming in.
Petrol stations aren't charities, and there's so little margin on a litre of petrol they don't take any chances of being caught with their pants down.
As far as the falling prices go - supply chain economics.
It takes a while for cheaper stock to get down the line to the forecourt. Of course there are some greedy sods in the chain but there always are.
If a litre of petrol costs 150 and the garage sells at 170 when the price goes up to 190 they have still only paid 150.
The argument is they don't put the price down of the fuel they have in their tanks. That they will pay 150 to replace.
They wait till they buy more to reduce the price.