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I think there may be some confusion here? Assuming that residue(its never that simple) is an accurate indicator of fuel additive levels, the results shown only indicate that different oil companies use different levels of fuel additives. They also will use different formulations for their additive packages. And it is not uncommon to put more additives in the higher octane fuels, to further differentiate the value for higher octane fuels than just octane. Fuel additive packages are a key differentiator of different brands.
I would bet this list is not intended to be a measure of consistency of octane of 91 & 92/93 octane fuels, but rather to show that different suppliers do vary the additive content between octane grades.
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