Refiners are still blending gasolines as if it was still 1970, with carburetors. Back then, there were no controls on cars to compensate for altitude, and no knock sensors. It was, and still is, common refining practice to blend gasolines intended for distribution in higher altitude areas to a lower octane specification than gasoline intended for distribution at lower altitudes. So it was common in many places to see 85 octane regular, and 90 or 91 octane premium. And it still is. As for California, the strict limits on aromatics make it hard to blend 93 octane fuel at reasonable prices, so fuel sold there is mostly 91.
In fact, long ago, it was common to see 91 octane premium even on the East coast at sea level. But as time marched on, refiners invested in refinery units that allowed them to blend higher octane fuel, so 93 is now typical.
By the way, in the old days, when oil companies advertised their octane ratings, it was the "Research octane" rating, which is typically about 10 points higher than the "motor octane" number. By federal mandate, the number now posted on the pump is the average of the motor and research numbers. So 93 octane today would have been advertised as 98 octane back in the 1960s. The highest research number octane commonly sold on the East Coast back in the 1960s was Sunoco 260, which typically had a research number of 103 to 104. Most premium fuels had research numbers from 99.5 to 101.5. I was a young engineer working for Exxon at that time, and had access to this industry information. Those of us with muscle and high performance cars loved having access to that information. Believe it or not, some of the highest octane fuel back then was sold by the "no name" brands, as they felt they were held to a higher standard by their customers, and did not want their customers leaving them because of complaints that their cars knocked on their gasoline.