And the explanation for this is given in Pilot's Notes General (1st Edition 1941, not the 2nd Edition).
http://forum.1cpublishing.eu/attachm...0&d=1334727256
http://forum.1cpublishing.eu/attachm...1&d=1334727263
We know that the Merlin II and III was designed for 87 octane and therefore the operational limits are always given for 87 octane.