A ratio is the result of a division, if you have 5 kills and 2 deaths, the K/D ratio will be 2.5 (5/2, or 5:2)

If K and D are both positive (or zero), the ratio can be positive, zero, or not exist (5/0 doesn't exist, 1/100 is still positive).

A difference is the result of a subtraction, if you have 5 kills and 2 deaths, the K/D difference will be 3 (5-2)

If K and D are both positive (or zero), the difference can be any integer number.

So having a positive ratio doesn't mean anything (I know, it means you died at least one time, but who cares ?). Dont use the word ratio if you don't know what it is, and please don't say it can also be a subtraction.

(Sorry for my bad English, it's not my native language)