The few times Hitech posted the hit counter results for a player who questioned the outcome, it looks like each plane is mapped for hit areas and has a count down counter for a certain number of hits or (weight of fire??) all over the plane. Each mapped part then will fall off or start smoking\burning when the number for it is reached. We know that point when a wing falls off, the engine stops or smokes and so forth. Then the magic BB to the pilot's head one shot kill and pilot wound.
A single round like the 20mm moves that counter farther in one contact than the .30 or .50.
This is my impression of what I've seen posted by Hitech, I'm not trying to speak for him. I could be bassawkwards on it.