MajTom has the right idea. Basically think of each part of the plane having some "value" as he mentioned. When you hit his plane, your bullets do damage to those parts by reducing the value. So lets say I make a pass on a B-26 with my Jug, and I do 2 points of damage to 10 different parts. I did 20 points of damage overall, but I didn't reduce any of the parts I hit to 0 so they didn't break or show any damage. Then you come in behind me and do 10 points of damage to his right wingtip, which breaks off ,and he crashes into the ground. I get the kill, because I did 20 points and you only did 10 points. Sure, you provided the proverbial "straw that broke the camel's back", but I did more damage overall.
Now I am just making up numbers to illustrate the point here, but I think it illustrates how this works.