I don't think I've ever seen 89 octane premium. I live right near the edge of where they make the change over in Montana. Where I live it's 87 regular, 89 midgrade, and 91 premium. But if I drive about 80 miles east, it's 85 or 85.5 regular, 87 midgrade, but usually still 91 premium.
I always assumed this was because on a car with a turbocharger, the turbo can compensate for the altitude.