A majority of major-league bullpens include seven pitchers. The Diamondbacks have nine, a number that will decrease when starter Patrick Corbin returns.
Even with a deeper relief corps than most teams have on their active roster, nine major-league-capable relievers are nowhere near enough to get through a 162-game season. Reliever performance year-to-year is just that unpredictable.
Over a 60-inning season, a reliever can either be disproportionately lucky or unlucky when it comes to balls put in play. Pitching so often leads to lingering or season-ending injuries, and short outings make it hard to iron out mechanical issues that arise.