Spot buying.
The "name brand" stations like BP/Amoco, Mobil, and Shell all own and operate their own oil field, refineries, and every bit of equipment from the time the oil comes from the ground up until it gets put into your tank.
Companies like ARCO, Clark, Marathon, and others buy gas on the spot market. It might come from any number of refineries or oil fields. They basically play the market. Most of the time they can sell for cheaper, albeit not usually as high of quality as the name brand stations. However, you will notice that on the occasions that gas prices shoot through the roof, sometimes the places like ARCO are actually more expensive 2-5 days later. It's again because they are playing the spot market.
