I was of the understanding it had something to do with the usage of a plane, but I don't think that is possible. Is it based on percieved performance and usage? Just for example, here are some things I find odd.
La-7 39,606 kills Eny 15
P-51D 37,516 kills Eny 10
Spit IX 41,764 kills Eny 10
109G10 12,159 kills Eny 21
190D9 14,467 kills Eny 18
110G2 7,743 kills Eny 20
So, the 109 gets about 1/3rd as many kills as the La-7, and is about half as good a plane... and the ENY is 6 points higher. The 190D9 also gets about 1/3rd as many kills, isn't even in the same league as an La-7, and is 3 points higher. The 110G2 actually gets LESS kills than the 109 does... and has a lower ENY. Furthermore... why is the P-51s ENY lower than the La-7s? It isn't anywhere near as good a plane, and the usage is about the same. Shouldn't they both have the same ENY?