I know you've said you played a large majority of the COD games (or at least I think you said that I could be wrong)
It's all about numbers man.
Eventually your numbers get so high that it basically requires doing at least .1 higher then what you're K/D is a fair amount of times to get it to go up, while 1-2 single games below that will drop it.
It's a lot easier to decrease an average then it is to improve one.
Early on it's no unusual to see the numbers flip a lot.
It really depends on how you are to begin with. If you don't have a ton of deaths compared to kills a handful of bad games will drop it 1-3points but it's going to take triple the games of doing above what you're at in order to raise it back to where you were.
EoTR- (PS4 Message me here if you want to party up)