Actually, your overal score for, for example, fighter category is the result of your K/D, K/S, K/T, HP and points ranks.
With this criteria, you may start a tour and fly a single fighter mission (F4U1C

), go to a vulched base and achieve impressive results in K/D, K/S, K/T and HP. That is, you return to you home base with a 15/1 K/D and K/S, .0040 K/T, .2500 HP and some landed points. Since them, this pilot dedicates his time to attack, buff and vehicle missions the rest of the tour, being at first possition in K/D, K/S, K/T and HP, and having one of the worst scoring in points. What is the result? Almost sure he will be into the top 100 fighter pilots.
The conclusion is obvious, all except points are very easy to achieve in a single mission. What about calculating your score only after you have achieved a minimum of points, or a minimum of sorties, or both? IMO, a minimum of sorties would be the best solution for any category (fighter, attack, bomber or vehicle). what about 15 sorties?
In the other hand, there is no reward for killing "skilled" pilots. You may go near an enemy base, and keep killing that alone rookie over'n over. What about extra points based on the ranking of the victim relative to your own rank? That is, you are into the top 100 pilots and you kill a pilot listed at 1200 pos, you get the normal point quantity. But if that pilot kills you, then he receives bonus points up to twice the normal quantity.
Same with the countries with inferior numbers, more bonus points per kill.