Tyler Clippard has emerged as the go-to-guy in the Washington Nationals bullpen. He currently leads the 'pen in IP thanks to 14.1 IP in June. He is third in FIP and fourth in xFIP on the team, mostly due to his lucky HR/FB of 4.3% which is far below his career line of 9.1%.
Inexplicably, Miguel Batista has the second most IP in the bullpen. He is the 5th worst reliever in the Majors in terms of xFIP out of relievers with 30 IP or more and he was the team's worst reliever in June. He has, however, been the best option for the Nats this month on the strength of 9 Ks in 5 IP. Don't expect him to keep it up.
Despite his three blown saves in June, Matt Capps is pitching well as he continues to keep his walks low (0.63 BB/9 in June and July) and Ks high (5.65 K/9).
Drew Storen and Sean Burnett have Tyler Clippard disease, which is a very good disease to have indeed: both pitchers have low HR/FB rates (0% and 4.2% respectively) explaining their deflated FIPs and ERAs. I doubt they will both continue to pitch this well for the rest of the season, especially Storen. Oh, and Doug Slaten has the same disease (5.3%) but he doesn't attract nearly as much attention.
The Nationals bullpen is a tough one to figure out. In June, the bullpen ranked 5th in terms of FIP and 22nd in terms of xFIP in the Majors. The difference is less extreme for the full season, 11th in FIP and 17th in xFIP, but it does raise an important question: Are the Nats getting lucky? This really comes down to whether you think HR/FB is something beyond a pitcher's control or not. The evidence seems to point to the fact that whether home runs get hit or not has much more to do with the batter than the pitcher. Whatever the case, luck sure is great when it's on your side.