For some people the kill/death ratio is kinda holy in Call of Duty. To proper calculate the k/d isn’t that hard – your amount of kills divided by your amount of deaths. But you need to take your total body count in consideration.
If your total were 10000 kills and 8000 deaths, your kill/death ratio would be 1.25.
Now, let’s say you’re going 1.00 (20/20) this would actually slightly drop your k/d, even if it wouldn’t show on three digits. That is, it wouldn’t show the first few matches going 1.00. Also, if you were to kill and die twice as much (40/40), this would drop your k/d ratio even more.
To improve your k/d ratio, in this case, you’d need to do better than 1.25. A match when you’re going 40/20 (2.00 k/d ratio) would slightly improve your overall k/d ratio.
Bigger numbers would render greater effect on your ratio, both up and down. This means that if you were to really beast with 100 kills and 50 deaths (still 2.00 k/d ratio), you’d reach a little higher than with 40/20.
P. S. I have many hours on Black Ops II but rapidly dropped 0.04 in a few weeks, maybe because I got bored with the game and tried to put right by challenging myself – using sniper rifles only.