FWIW, the Shell data also noted that unleaded fuel was particularly 'sensitive' to the addition of tetraethyl lead, meaning a small amount added resulted in a disproportionately larger increase in octane rating.
THAT EXPLAINS IT! Now it makes sense.
I copied a Consumer Reports article back in the days of the demise of leaded premium. They claimed that a 50/50 mix of Shell 89.1 leaded regular (still available then) and 91.6 unleaded premium would yield an octane rating of 91.2, much more than the average of the two.
