But that's a local competitive effect most of the time. Stations in a given locality tend to all sell at roughly the same price, because "gas is gas is gas" and if the guy down the street's selling at $2.45/gallon, you can always drive another half-mile and find it a dime cheaper. How many times have you sat at a busy intersection with two gas stations on opposite corners selling for the same price? If one lowers their price, the other's forced to do the same to keep customers...
That's also why they come up with gimmicks like "Mobil Speedpass" or whatever it is...they have to establish brand loyalty and a competitive edge somehow, and if gas is gas is gas, the guy with the Mobil Speedpass is going to favor Mobil over the Sunoco across the street.