Probably a bit of both. Gas is often more expensive in more well off areas, which also usually have fewer gas stations in general, and the rents those stations pay are probably a bit higher too. It's also more near airports, because demand is high for rental car returns.

But part of the higher price is a function of your willingness to drive to a poorer or less convenient part of town to fill cheaper gas in your car. As a younger person on a limited budget I would do that frequently.

But depending on how far away the cheaper gas is (meaning gas used to get there), and the value of your time spent getting there, you might be better off paying a bit more to fill up closer to where you live. That's what I do now, even though it's a $.20 more per gallon.