Why does the Elo I gain from games fluctuate so heavily?

When I am playing on Chess.com, the amount of elo I gain or lose from each game seems to be randomized, due to the fact that I gain less and lose more elo, when playing against players with a higher elo rating than me. But when I am playing against players with a lower elo rating than me, I seem to generally gain more and lose less elo.