You mean that if I fly a C.202 - and I get a kill, I get more perk points than if I fly a Tempest?
Hmmm... let me think about this...
Let us say that you're flying a A6M2 and shoot down a P-51D, you'll get more perks for killing a late war fighter. If you were in a P-51D and shot down a A6M2, you'd receive less perks for killing the early war Zeke than you would if it was the other way around.
Early war plane shooting down Late war plane = More Perks
Late War plane shooting down early war plane = less perks
ack-ack