it’s because our midgrade has constantly been 89 and premium was 91 in many parts of the country.I always thought US 91/92 was Euro regular, seeing for years many BMWs/Mercedes call for premium and VW/Audi for the exception of the older 1.8T/VR6 and Audi 2.8/3.2/4.2 engines regular in the US/Mexico.
90aki is enough for 99% of the engines on the market. if you have time on your hands you can mix it up yourself