Mission Rating??

[jinkazama]

I completed the first mission, I went around killed a few Nazis and completed all the objectives. Upon exiting the level I got a Professional rating and about 3200 points.

But the rating bar is not full. So I thought to myself, maybe I shouldn't kill anyone, just stun them instead, that should net me better rating. So I started over and completed the first mission with 0 kills. Guess what, I got a Scout rating and about 250 points.

LOL, can anyone tell me how the game mission rating works?

much appreciated