I just thought of something today about octane enhancers and the different grades at the pump. We know that ethanol is an octane enhancing fuel additive. We know from the labels on the pumps that "all fuels may contain up to 10% ethanol." The key words being "up to," but it doesn't also say that 87 octane may contain more or less octane enhancers than 91-93.
It would make sense that the simplest way to modify Premium to have a higher octane would be to increase its ethanol percentage. And for Regular octane fuel, keep its ethanol percentage to a minimum.
So for instance, 91 octane may actually contain 10% ethanol and 87 octane might have something much lower, say an ethanol percentage of 4%. I've searched the Web, but cannot verify my hypothesis. The one that eludes to it, CarGuys, doesn't give a percentage for premium nor a comparison to the amount contained in regular. I'll keep looking from time to time. Seems like a needle in the haystack question at the moment.