Yes, I am aware that named brand gas stations can also use fuel from
other companies. I was told that one of the reason has to do with the
company insufficient distribution network in a local market. If the
oil company has a large retail network but their distribution network
cannot keep up, they will have to buy fuel from other companies
(cheaper this way than building up distribution capacity). Many major
oil companies do this. The quality of the fuel that get pumped into
the gas reservoir in the gas stations really have very little
difference; the difference is how well the chain manage their retail
stations, and their marketing power.
Thanks for pointing this out because many people probably don't realize
this (I didn't know this until an oil analyst told me).