That definition sucks anyway.
Just go with "has an advantage". 10-0: unbelieveable big advantage that makes it nearly impossible to win.
5-5: even
6:4: one has an advantage, but it's not too big
etc
If one character has an advantage, he should win that MU almost 100 % of the time. I could give it the rating 7:3, because it's a big advantage, but for you it would mean "could still lose the set, even though it's unlikely", even though it is very very much more unlikely than that (unless the player is just much more better).
This is what the -4 to +4 rating is trying to do. It wants to show that it's not a ratio, but imo there are too few options for them... but we all know it might not even help, because people have such big different opinions that some think a MU is +2 and others think it's -1 or w/e. This could make it even more difficult to come together to a ratio...