I've heard the theory that there are "districts" in the petroleum market. They are not official, they just sorta happen. Within each of these districts, there will be found one "alpha" fuel dealer. He will own a distribution network to gas stations and bulk dealers that allows him to dictate the price. Should a smaller distributor in his district go against him by selling their product at a lower price, he will simply lower his prices further yet, effectively breaking the smaller dealer if he doesn't relent and get his prices back up in line with the "alpha" dealer.
It happens plainly where I live.