Fantasy football isn't the most predictable game in the world. If you wanted to play a game that lacked variance, you could always go with, I don't know, chess. Or checkers. Or something way less entertaining than fantasy football.
Since things can be a little random in fantasy football, crazy, out-of-the-ordinary weeks can happen. Things like Marvin Jones scoring four touchdowns in a single game can occur during the same week where David Johnson is active, but Chase Edmonds steals 27 of the 28 running back rushes in the Arizona backfield.