Usually due to franchising and monopolisation.
The big name stations are usually owned by a franchisee who wants to make a profit on their investment as they can only buy the gas from the franchisor for a certain price.
They then price based on profit margins so a station with a higher customer number will be more likely to charge less as they can still turn a profit.
The smaller noname place is not stuck buying from the one place so can negotiate and look around for better prices so can charge the customers less.
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