You're missing my point entirely, taking it out of context, and going back to generalities which are too broad for claiming one brand can offer better gas mileage than another. Such broad generalities don't advance the specific topic I was addressing, which is gas mileage differences between brands.
Since base gasoline is a known common commodity, differences between brands is primarily the additive package. These packages supply octane modifiers, detergents, corrosion inhibitors, antioxidants, and oxygenates which are carefully controlled by all brands, especially the Top Tier brands. The octane rating must meet specific national standards to be rated at a specific number, so if one brand is rated at 92 octane, it should be more or less equivalent in octane to another brand rated the same. So that negates that specific factor which could affect gas mileage. Additionally, unless a specific brand claims that they do not add 10% ethanol (E10), which lowers the energy of fuel, that gas mileage factor too can be negated.
The rest of the additives should not, in the short term, provide significant gas mileage differences between brands. If they could, and one brand had a proven, exclusive, significant advantage over another, believe me they would advertise it profusely. In the long term additives might make a difference if say the one with more detergents kept your engine significantly cleaner. But for someone to accurately claim that they got noticeably better gas mileage from one tank of a specific brand over another is simply conjecture because the factors that would give gasoline significantly more energy from brand to brand is very small, and it would take far more accurate, long term, and controlled testing to conclusively prove any differences.
This is why I find it very hard to believe anyone who states offhand that they get better gas mileage from brand ABC than XYZ. Until an accurate, well controlled test done by an accredited source become available, don't believe such claims. The only generalization that can be reasonably accurately stated is that Top Tier gasoline is probably better to use because the additive package must meet specific standards to claim a Top Tier rating.