When I used to work in Havant, And I used to deliver parts to Littlehampton early morning or on my way home ( acted as an unpaid motorcycle courier!!!) I used to pass 4 Tesco garages, and on loads of occasions, the price at each one on the same day was different.
Havant was usually the cheapest, Bognor and Littlehampton about the same but Chichester was always the most expensive, Up to 5p a litre difference.
When I queried this with the staff, they could never actually give me an answer. As I used to say to them, they all sell the garage shop merchandise at the same price so why should the fuel be any different???
So basically it’s what they can get away with based on other local petrol station prices.
No different to the Rip Off Motorway services who seem to be able to charge what they like.