Gasoline pumps here in the states post octane numbers as an average of two different values, quoted as (R+M)/2. The R stands for research octane number (RON), which is determined with a test engine running at a low speed of 600 rpm. M stands for motor octane number (MON), which is determined with a test engine running at a higher speed of 900 rpm. If, for example, a gasoline has an RON of 98 and a MON of 90, then the posted octane number would be the average of the two values or 94. In some other countries they only quote the RON figure which is why in Japan for example they have 98 RON gas but in reality it's no more resistant to knock than the 93 (R+M)/2 stuff we get here.

Any car sold in the USA that says in its owners manual or on the dash that it requires premium gasoline is tuned to run with a minimum octane of 91 (R+M)/2 because that's the best available in certain places like California and Colorado. The reason JDM versions of engines sometimes have like 10 hp more than the USDM version is because the JDM version are tuned to run on what is the equivalent of 93 (R+M)/2 gas. It's available over there everywhere, there's not huge swaths of the country (i.e. california) where the best you can get is 91. So it's not that pump gas over in JDM-land is any more exotic, it's just that they don't have areas where the premium is somewhat less premium than other places' premium.