Gasohol

How can "pure gasoline" coming out of a 'supply depot' have the same 87 octane as the same "pure gasoline" mixed with
10% ethanol? Since all auto gasoline is coming from the same distribution pipes, what is the octane in those pipes?

Folks have said that the various sellers have their own additive packages, and others have said that the ethanol is
added near the point of use, and still others have indicated the ethanol is added to increase the octane rating. If all
this is true, then the gas in hte pipes could be some low octane rating which is then boosted with ethanol to 87 octane
for the pumps. That infers to me that even if you bought gas straight from the pipe it would not be 87 octane. Not good
for STC holders...