Are you disagreeing that relievers can't be volatile within the season? If so, have to not seen Kimbrel's stats? In case you haven't let me break his season out for you.
Cubs - 39 games. 0.49 ERA. Allowed ERs in 2 of those games. 1ER and 1ER.
Whitesox - 13 games. 6.57 ERA. Allowed ERs in 5 of those games. 3ER, 3ER, 1ER, 1ER, 1ER.
Happens when they don't even switch teams. Devin Williams. He was 2nd ranked RP in WAR last year. First 22 games this year he had a 4.22 ERA, looked lost at times. Over the 31 games since those first 22 he's had a 0.31ERA.