Depending on location most cities have bulk storage depots. Sometimes they are owned by the big refiners like shell etc. Sometimes they are operated by larger conglomerates you likely have not heard of. The fuel comes to the depot by pipeline , ship, truck depending on volume and location. The depot is also where the fuel trucks come in to load. Depots can serve several customers like Chevron, BP, shell etc. When truck hauling a Chevron load comes in and hooks up to load the additive is directly injected into the fuel while loading by computer system. The next truck could load a completely different customer next with a different additive package injected. They may all use the same base stock fuel. Some companies lease tank space and bring in their own product but that is pretty rare. Most bulk stock comes in on pipeline so having a unique fuel is almost impossible.
There are fewer and fewer refineries so unique base stocks is really not an option. We see a bunch of marketing hype making us think we are getting a unique product. I do agree that all addative packages are different to some extent and some are better.
Here in Michigan Marathon Petroleum has their own refinery and distrutbution center, trucks, etc. They use STP additives. They only cover MI and the surrounding area, so I think getting fuel from them it is going to be theirs and theirs alone.
But I get what you are saying and I think you are correct. Octane, cleaners, etc. are added to the truck and any extra left in the truck after deliveries is sold at a discount to the no name discount who knows what you're getting gas stations.
I have heard of gas stations loosing their franchise because they had gotten a fuel delivery of the wrong brand.
I know with motor oil the base stock is all the same (except for true full synthetics) and the additive package is what makes the oil's different.