I would guess United have had more shit buys so it might be a bit skewed due to that. It's not like you didn't buy in the same period (since Aguero also counts) Rojo, Blind, Herrera, Schneiderlin, Di Maria, Schweinsteiger, Depay, Darmian, Mkhitaryan, Lukaku, Dalot, but I don't even see Diallo listed in the above list. Not to mention the astronomical wages and transfer fees to the likes of Ibrahimovic or Alexis or Falcao. Obviously City have had the likes of Bony, Jovetic, Nolito etc. too. So of course United spends comparatively. That was reflected in the respective wage bills throughout the seasons.