I personally don't get it I mean I got lucky and got something or you did the weeklies saved up your coins and got lucky that xur was selling it either way you look at it this whole thing involves luck. Unlike mmos this game doesn't have bosses with set loot tables that have percentages of when and how a item will drop. So regardless of who you kill during a roc or nightfall or whatever inside a raid it's purely luck based when it drops.