Octane ratings are 4 points from what they were back then for starters, there is a research method and convention method of testing, so the scales differ 4 points. Also LEAD techraethyel lead makes the engine cooler in the combustion chamber too.
Todays cars run hotter to burn cleaner, the cams are smaller so you end up with more static low end compression, hence less wasted fuel out the exh less= les smog.. egr valves etc etc.