Many things can make a difference. Simply thinking about economy can alter your riding without you noticing. A tailwind can make a huge difference on a bike. So can type pressures, time of day as in how much traffic. Even temperature, because carbs measure fuel by volume, cold or hot fuel & air can produce different power levels. Combine all of the above & you will see why manufacturers use machines to work this out, no variation.
As always YMMV !
