That Livan Hernandez has been extraordinarily lucky this season has been well documented. Though his last two starts have hurt his stats, his LOB% (88.8%) is still ridiculous. And then there's that difference of 2.75 between his xFIP and his ERA...
Craig Stammen is the best regular starter the Nats have despite his 5.60 ERA. His xFIP of 4.09 is the lowest out of regular starters and he is being hurt by his very low LOB% of 56.6%, which is probably largely attributable to bad luck, though Stammen does have a rather low K/9 at 4.28. At any rate, his ERA should come down soon.
John Lannan is not having a good year: 15th worst xFIP, 3rd worst K/9 rate, and worst K/BB rate in the Majors amongst starters with 30 IP or more. It seems that Lannan has lost some horizontal movement on his fastball and we'll have to wait to see if it will come back. In the meantime he can stop dishing out the walks (4.20 BB/9).
Luis Atilano has the highest xFIP on the team and 12th highest in the majors (5.62). Without his worst start, however, Atilano's ERA would be 3.82 and without his worst two it would be 2.60. Because he's young, Atilano can be given the benefit of the doubt for now but he too would be well-advised to start cutting down on the walks (4.58 BB/9), especially considering his rather slow fastball.