The idea behind it is that if you're doing great, your team will win. I know there are some exceptions, but if you have a perfect game, generally your team will win. This means that (I'll use me as an example) if I'm mid (since I play mid a lot), it's not just about winning a lane. It's about using the advantage from winning my lane to help the other lanes. If I just spend all the time in the mid lane and don't leave, I'm padding my stats, but not helping my team win. So yes, there are times when you play great but all 4 of your teammates are terrible. It happens. But more times than not, if you are playing great and use that advantage as you go into the mid game to help teammates and team fights, you'll win.