If you need only 5 to 8 rounds for 1 kill, that's FAR from being crappy: 12.5% - 20%. Or 8-12 kills for a full 65 rounds loadout.
Is the score a percentage value from the total ammo load out or rounds shot? I'm assuming it's rounds shot that. If so the missed shots would add up over the course of a tour. The 1 kill per 5 to 8 rounds shot is not a real in game example, just numbers I used for example.
Also even if someone averaged 1 kill for every 5 to 8 shots it doesn't mean they would land 8 or 12 kills.. It means they took their time and set up the right shots. This likely would mean they would likely not land a hell of a lot of kills. You just don't have the fuel load out in that plane to land that many kills if you are taking the time to set up kills to keep a good percentage.
Not unless they vulch, that is.. Which is another crutch that can be used to pad the hit precentage score.
The point I was making, is by flying a plane able to put more lead in the air, you can skew the score by the amount of ammo you are able to shoot. You can get many more hits and over the course of a tour that is going to average out the misses for you.
Meanwhile if you are flying a single shot plane, you simply can not put the same amount of lead in the air, which makes each miss more of an impact over the course of a tour.