Most contries measure octane in RON. The United States uses the anti-knock index (AKI) which is (MON + RON) / 2. The MON and RON numbers are measures of how much you can compress the fuel before it combusts(with no spark). One is under heavy load and the other is under light load. RON is higher than MON and hence higher than the AKI. The reason you see fuel in Europe and Japan with 95-100+ octane is because they only use the RON rating instead of averaging it with the MON rating.