Yes it can be. It also varies between induction strokes and varies between cylinders in some cases as much as 11-1 to 17-1.
Its the vaporization rate not the octane. Lower octane fuels are designed to vaporize in low compression engines. If its lean at time of ignition you haven't converted enough of the fuel to a gas state around the plug for it to get the process started efficiently. You only burn what transforms from a liquid state to a gas state. Notice those stocker plugs all had colour on them? The OEMs spend billions to get the combustion right.
