[Allch Chcar]

That's a good point. I only looked at the E85 octane numbers and ignored E50. There is no logical reason that the two mixtures had almost exactly the same octane. The other results are of a large enough different between E50 and E85 that I suspect it may have just been the Octane test that is strange. I've sent them an email to see if there is more to the story.

The Octane values for RF Gasoline and High Octane Gasoline are commonly accepted and well published numbers. RF Gasoline is a very common and regulated product. The only variables I can think of between Gas stations excluding water content(which is regulated) are the aromatics, dye, and detergents. All three are variables that may not be large factors for Octane.

Alcohols have oxygen in them which reduces their energy density but also means they can tolerate much higher Octane than Petroleum products. Pure Ethanol is 120-130 RON. Gasoline was mixed with lead in the 60's to increase it's Octane but was later banned in the US due to Lead being toxic, even more so than Gasoline. Other than adding Lead there are no cheap options for improving the Octane of Gasoline that I am aware of.

Edit: Okay I already found it.
"It is interesting to note that the blending response of RON
and MON as a function of ethanol content is highly nonlinear.
There is a substantial octane improvement between
RG and E10, and between E10 and E50. However, between
E50 and E85 there is very little difference in either RON or
MON. It is well-established that properties of ethanolgasoline
blends, such as Reid vapor pressure, do have nonlinear
responses [4]. Surprisingly, the authors have been
unable to find RON and MON measurements as a function of
ethanol content that either support or refute these results."

It looks like the link for the source is restricted.