They are all selling the same gas from the same refinery. They might have slightly different additives but otherwise it all comes from the same place. you can't afford shipping costs to get gass from a different refinery (unless you are on a territory border)

the only question is how much profit to put on top and even then they have to be careful as if they go too low their competetors won't follow and then they have to pay extra for an unscheuled delivery just to fill their tank.

i'm reasonable certian that small stations are illegally colluding in some cases.