KDA doesn't determine your 'real power'. Gold and level does.
4/0 means jackshit if you don't have the gold. Look, I may be the 0/2 Master Yi adc with a botrk and a zeal, but that 4/0 jinx with only an Berserker's greaves and runnan's isn't a threat to me because she didn't recall yet.
Same thing for lategame. You, the 100/0 lux, isn't gonna be able to fight be the 0/100 yi if I have full build and so do you. It's about the gold, not the KDA.
League is like winding clocks. Getting a lead is giving you more 'time' on your clock, but powerspikes are like hours. Until you've passed the 'Hour', even if you're at 2h50 and the other person is at 2h30, you are about at equal grounds until you're at 3h.