supposedly, the higher the altitude, the less your car needs the high octane due to the engine retarding its timing to account for the lower amount of oxygen in the air. at least thats what i remember from the last time i looked it up. its not that high altitude somehow magically increases your octane
with that said...at least you get 91 octane up there...im nearly at sea level here, and we still get 91 max. damn california and its strict emissions!