This article is from the Gasoline FAQ, by Bruce Hamilton with numerous contributions by others.
RON - MON = Sensitivity. Because the two test methods use different test
conditions, especially the intake mixture temperatures and engine speeds,
then a fuel that is sensitive to changes in operating conditions will have
a larger difference between the two rating methods. Modern fuels typically
have sensitivities around 10. The US 87 (RON+MON)/2 unleaded gasoline is
recommended to have a 82+ MON, thus preventing very high sensitivity fuels
. Recent changes in European gasolines has caused concern, as high
sensitivity unleaded fuels have been found that fail to meet the 85 MON
requirement of the EN228 European gasoline specification .