sort of a way to "class" different planes
la7, spitIX, things like that have low enemy values because in theroy they as less "diffucult" to fly, or can perform better than others.
a spit IX VS a C.202 is not a "fair" (or historical) matchup, hence there has to some "advantage" to flying the 202.
thats where you get perks. if you are skillful enough to kill a spit IX in a 202 you will get tons of perks. it takes less pilot abaility to kill a 202 in a spit IX, there is nothing the 202 can do that is superior. hence killing a 202 in a spit IX is no challenge equaling less perks.
all examples are based on 2 identical twins flying against eachother who have identical flying skills, on identical systems, with identical setups, on a closed LAN play network with perfect connections to each other, while both planes flying identical altitudes, and speeds.