That would be the most unlikely scenario, I've done training for Exxon, Arco, Mobil and Union oil for the pipelines moving the gas and also training at the tank farms. The fuels are mixed before going into the pipelines to the tank farms and trucked to the stations. No national brand allows a distributor or station to modify their products.
I have over 40K of my 50K miles with my FiST Mountune on the highway. Will add about 6K more later this year. I have done 2k in past 3 weeks. Shell sucks in MY car no matter where I have bought it. In the past 2 years I have been to over 20 states and working on my 2nd time of driving all 48 contiguous states. I believe it is the additives and I doubt any Shell branded station sells diluted gas nor do they sell non Shell gas, LOL! Small companies may play the games you talk about but no national brand will allow tampering of their branded products. Some refineries will produce special mixes for small companies like Midnite Oil here gets my favorite 93 octane non-ethanol from Marathon. I don't know of any national brands that sell it normally. What my post said I will repeat, Shell does not do well in MY car and I don't buy generic gas. Average Mpg is displayed on my dash at all times and reset with fill-up when on the highway. Since one of the reasons for buying the FiST other than tearing up mountain roads was its economy for my wanderlust ways mpg is important to me. Every buck I save in gas goes to enjoying the journey. All my mpg comparisons were in those miles I use for testing and are in the Shell S.E. region. It amounts to over 8 round trips I take to Augusta, GA from Chatt.,TN each year. My route for observing mpg is repeatable and consistent.
BTW, my wifes Mustang GT Coyote has no adverse reactions to Shell, my sons 650 HP resto modded Trans-Am nor my daughters stock FiST seems to care either. So I am not a Shell hater, my car is. I have zero problems with filling the Mustang with Shell. Shell claims to be the #1 preferred gas in North America, I have no reason to doubt that as it seems to work well on other cars, but not mine. If it works for you buy it, but try to understand I have seen multiple times that it doesn't in my car and the reason I buy only top tier branded gas is I know the formulation has not been altered like an independents can do. As far as I know all national gas brands keep their additives proprietary mainly by patenting and copyrighting the blend like Shell's "V-Power NITRO+" or Chevron's' "Techron" so comparing to get an idea as to what is going on as to additives would be a lost cause.