The name brand stations don't always sell the fuels coming from their own refineries. Gasoline is pretty much a commodity item these days, supplied by a vast network of pipelines throughout the country. If a refinery for a perticular oil company is not nearby, then the particular oil company buys from whoever can supply the demand.
Shell, IIRC, only has two refineries (with a few joint ventures); they can't possibly supply all of their stations across the country. Also, a lot of the private brands come from the big boys also.
The bottom line - what you think you are getting, may not be what you are getting.
How do I know? I work in engineering for an independent refiner.
