Mani, exactly correct. The same gas gets pumped up the same pipelines to the same distribution centers & is pulled from the same tanks. Here in Cent. & Eastern WI all of our gas gets pushed up from Chicago via the same pipeline. Hence the reason why ethanol will never be the solution. You cannot pump ethanol through the same pipelines; you have to create a whole new infrastructure just to ship it.Though there is some kinda bizarre allocation method the pipelines use essentially it's all the same gas.
Most stations do have at least 2 separate lines from the tanks to the pumps; one for regular, one for premium. The mid-grade stuff is mixed at the dispenser.