I wouldn't trust Sunoco's claims for octane at all. As for why, plain old experience.
This is what I used to tow my boat from Detroit to Lake George, NY and back home for summer vacation. Boat and trailer weighed in at about 5100 lbs. With the original 305 it was VERY octane sensitive so I installed an Edelbrock Water Vapor Injection system to control knock. Usually ran with about 30% methanol in the tank so that it wouldn't freeze in the winter. Also found out that methanol really helped with charge cooling. Note, also ran a weight transfer hitch. Anyhow with Amoco, BP, Sohio, and shell gasoline I ran a rather light setting for the mist. Every time I hit the first fuel stop on the New York Thruway it was always GD Octane Cheating Sunoco. Every time I filled up with that crap it would sound like marbles in the engine when trying to get up to speed. So I'd have to pull over, pop the hood, and crank the mist control all the way to maximum. If there was room in the water tank for it I pour in a quart of Methanol. For darned near 20 years any time I had to buy Sunoco on that Thruway I'd have to readjust to mitigate knocking. If you want to get ripped off, pay for premium, and get 86 octane then stop at Sunoco. BTW, wrote letters complaining about this crap to the NY DOT and Sunoco corporate and got nothing but crickets.
Yeah, I know. Not really relevant to this thread topic but consider this a warning, Sunoco is one of the worst Octane Cheats on the entire planet. If you want fuel that actually provides the octane posted I found Shell to work quite well. I'll also note that I sold the boat and my last trip to Lake George in 2019 was with my FiST and I did notice the power was a bit off with Thruway Sunoco in the tank.