Relievers, by their very nature, are quite volatile.

The really good ones are good year-in and year-out. That's why pitchers like Mariano Rivera, Dennis Eckersley, Trevor Hoffman and Billy Wagner were so good. They maintained consistency on a yearly basis. Cincinnati's nERD - the number of runs per game better that pitcher is over a league average pitcher - and Baseball Reference's Wins Above Replacement (rWAR), which I like better than FanGraphs' WAR for pitchers (fWAR) because Baseball Reference uses ERA in their calculations, not Fielding Independent Pitching (FIP).