Not saying every octane point = 1 %, but 91 Octane is just happens to be ~ 4.2% less than 95 Octane. I was just throwing out a ballpark figure.

Actually the compression in an engine is a constant, not matter how high the octane is or how low, the compression is the same. The octane rating however affects the timing of the ignition, with affects the duration of the power stroke which is where the power comes from. With a lower octane rating the ECU has to take out/retard the timing of the ignition to prevent the 'knock' (pre-ignition ignition), that in turn reduced the duration of the power stroke of the engine and hence lower power output.