Various components have various octanes; Today, inline ,computerized blending is generally used. In the "good old days" , a tank would be filled then tested. The interesting part is that the blending/octane is not linear; that is half of a 100 octane component mixed with half of a 70 octane component may give an 87 octane component. Then there is summer and winter octanes , plus high altitude and low altitude octanes; meanwhile keep the Reed vapor pressure in range. It was never as simple as you imagined.
And now ,either congress has a bunch of chemical engineers or they are buying corn belt votes with ethanol and are making specific requirements on gasoline. And in round numbers, for every gallon of ethanol put into gasoline , a additional gallon of oil must be imported (to produce the corn/methanol); Not to mention the 30% loss of energy/gal of ethanol vs gasoline.