If your playing this game for the numbers, forget it ! There are so many things involved it just isn't an accurate way to follow.
If you take the JU87 up with that big 4k bomb and drop it on a town you will take out oh say 8 buildings. Take the same plane and the same bomb and hit a VH. In both flights you hit, but if you hit the town on the first run, then the VH on the second run your percentage would drop.
8-1 , then a 1-1 averages to 4.5-1 see what I mean? So if ya do just the first run your % number will be higher, skip the second runs

On the other hand if ya want to bring you % up after doing a bunch of 1-1 runs, switch to the big bombs and hit multible targets.
numbers are easy to manipulate, thats why not many people bother with them. The guys at the top of the boards, for the most part know these "tricks" and use them to get there "names in lights". Play the game to have fun, if ya need to bomb something hit it and move on, don't worry about the numbers, they don't really impress too many people.