The only thing I can think of is the following:
Say you're kdr is 2.00
But in all your games you are getting say... 10 kills 9 deaths. That's not 2.00, but it is positive. So, you're 2.00 will still go down to ... 1.10 I think is what 10 kills 9 deaths is. It will go down, just not as fast as going negative.