The original Elo system has an uncertainty term, called the K factor.Elo rating system - Wikipedia It is larger for newer players and lower ranked players.
Overwatch also has an uncertainty term. It is larger for new and recently inactive players. “Play a lot of games, it (MMR) gets more certain. Don’t play Overwatch for a while, it gets less certain. The more certain the matchmaker is about your MMR, the less your MMR will change in either direction based on a win or loss” --Scott Mercer (Overwatch Forums). The statement, “You go on a large win or loss streak, it gets less certain” is no longer valid, as win streak bonuses (and loss streak penalties) have been removed from the game.
Overwatch’s system resembles Trueskill strongly. See How Competitive Skill Rating Works (Season 9) for a more complete explainer.