It's not KD. Bungie uses your Combat rating and something else to determine your skill. Being in the top 20% means that you have at least 30% of the player pool to draw from. The only thing you should notice is an overreliance on meta weapons. (re KD: I don't care about my KD. It's virtually unchanged since Y1. My issue is purely the lag+no rewards for being a top player. I entered the top 300 in Overwatch and instantly got a golden weapon for it 2 weeks afterwards (a weapon skin which usually needs 300 competitive wins to be unlocked)).