If your k/d in one particular game is lower than your overall k/d, your overall k/d will come down. For example - You play your first ever game of BO2 and your k/d is 4. You play your 2nd game and get a k/d of 2. Even though 2 is still a positive k/d, it is lower than 4, so your overall k/d will reduce to 3 after your second game, even though you got a positive k/d. Understand?