FYI - Not doubting that gas some station X may be better than station Y.
However, just so everyone knows, gas stations are independently owned, for the most part. ExxonMobil does not own gas stations with their brand names, this business was sold years ago, as they make much more money getting the oil out of the ground.
All local stations get gas from the same local gas terminal. Each gas station with a name brand then blends in an additive package, including detergents, etc., that is specific to the brand. Thus, the gas, if it is labeled correctly, should be essentially the same at most stations
It is possible that a given station or collection of stations are selling 89 as 91, which would of course be illegal and is why other posters are suggesting doing analysis and taking legal action if need be.
One final thought, are you filling at a pump with a single nozzle that is shared for all three grades? If so, if you fill up after someone who filled with 87, is your first bit of gas you are pumping 87 even if you choose 91? I have always wondered this, and do not know. This makes a bigger difference on a car with a small tank, like a Lotus, as a larger fraction of the pumped gas might be lower octane.
2001.5 Audi S4 Avant, Santorin Blue, Onyx/White, Stage 2+, Daily Driver.
2006 Lotus Elise, Aspen White, Touring Pack, Black Pack, Starshield, Larini Exhaust, Fujita Intake, Weekend Fun.