Also, the car manufactures have to put that in the OM because of the variety of climates and altitudes that the vehicles may encounter. What I mean is, if you're at sea level, the engine may like one octane. But, if you are at 1 mile above sea level, the engine may require another still.
Or as mentioned, the amount of carbon deposits that will build up on the pistons over time, thus, requiring the engine to use a higher octane to avoid detonation.
And although, engine build quality/consistency is at it's best today(I would assume), no two engines are exactly the same!