I had an MBA professor who was an Atlantic/Richfield (now ARCO) executive.
I learned that all refineries - no matter who owns it - sell their refined gasoline to a pipeline company who pumps it and stores it at a "Feed Tank Farm" strategically located in certain markets. Basically a cocktail of all the gasoline it buys.
They then pump and sell this cocktail of gas to "jobbers" for retail sales at a place called the "rack" - these are the trucks you see pull into the gas station to deliver the fuel. The delivery trucks (Jobbers) that service stations in your area all come from the same terminal or "rack" - over and over again. It's too expensive to do it any other way unless you live close by a refinery that direct delivers to its stations.
Only the additives are different or the gas station may "blend" on its own or has leaky tanks.
http://vettenet.org/octane.html