Whose fault is it this time, CARB? The EPA? The CHP? None of the above. This time we're being victimized partly by the oil companies, and partly--this is the one that hurts--by ourselves.
You see, when crude oil is refined into gasoline, the refinery doesn't have all that much control over what comes out. Crude oil is full of all kinds of stuff, and a refinery simply separates it, sorting all the iso-this and hepta-that in order of density. The really heavy stuff, like tar, is near the bottom, while the really light stuff, like butane, is near the top.
Somewhere in the upper ranges of the stack are the components of gasoline. There are between 10 and 15 different blend stocks, each with a different octane rating, which are mixed together to make gasoline.
The crude oil being used and little else determine the amount of each blend stock available for mixing. Generally, if you just dump all the blend stocks into a bucket, you end up with something around 88 or 89 octane. If you're selective and only mix the good stuff, you can make 92, 93 or even 95 octane. But once you take out the good stuff, you're left with crap--something like 85 octane. Then you have to leave enough good stuff in the bucket to bring this pee-water up to at least 87 octane. This limits the amount of 95-octane gas you can make. If you make 93-octane premium instead, you use up less of the high-octane stocks, allowing you to make a higher proportion of premium fuel.
In the Midwest, where an extensive customer base of good old boys in pickup trucks consume vast quantities of 87 octane, demand for premium fuel is low enough to make genuine high-octane premium.
In California, however, Lexus-driving executives suck down premium fuel like it's Evian, so 92 was the rule.
CARB isn't entirely innocent. Many of its standards for evaporative emissions and misdirected attempts at oxygenation have raised the manufacturing cost of high-octane gas, but it doesn't seem to be behind the sudden change to 91. Instead, according my super-secret oil industry mole, it all comes back to money. Unocal, you see, has a patent on the 173 easiest ways to make California-friendly 92-octane gas. As a result, every other oil company has to pay Unocal 5.75 cents for every gallon they make using one of these techniques. They haven't actually been paying it, but that's an issue for the lawyers to sort out.