After following those on the Top Tier Gas list for awhile, some of the firms do change out. Additives do cost money, and to save a penny or two at the manufacturing level, it can make a difference in their bottom line. I have no way of knowing how close some of the suppliers not on the list are to the magic cutoff, and they may be close, so that could account for the differences seen in that test you mentioned. But, at least around here, the difference in cost is minimal to chose one that is on the list, so I've been doing that 95% of the time. When on a trip, I make exceptions when in a strange area, and needing to fill up. Some of the no-name, local gas stations buy gas from various vendors, and their fuel might be good one delivery, and not so good the next...so I choose a brand with a corporate backing (I've been mostly using SHell).
If you notice, the auto manufacturer consortium that started up Top Tier Gas testing includes six big car producers...so, it's not just BMW that says buying good gas prevents issues, it's BMW, General Motors, Honda, Toyota, Volkswagen and Audi.