Barnes's career ERA is 3.93, but this year it's 2.61. I tend to throw out 2020 as an anomaly, a nonseason that had to be counted as a real season. In the three prior seasons (2017-2019), his ERA's were 3.88, 3.65, and 3.78, which is a pretty tight shot group.
This year is clearly different, not at all like those three good seasons setting up for the closer. And I think two things changed for Barnes to get 2.61 this year. First and foremost, he changed his repertoire to a very good knuckle curve, a good fastball, and nothing else. In prior seasons he threw other pitches which weren't as good. The other change was becoming the closer, which meant he usually knew when he would be called upon--usually with a clean inning. A possible third thing is that none of his set up guys (Ottavino, Whitlock, Sawamura, et al) throw his particular mix of pitches.