Both teams are roughly where they should be, though. 5 games over .500 is about right for +20. 1 game under is a little on the good side for a -15, but not by much. The Nats have not been unlucky so far this year in the record they have. They may improve and win more games in the second half, but their record stands about where it should given the way their guys have actually played.
Their rotation has not been good, and their defense is horrendous. Their offense is actually what's carried them so far. Now, you may argue their pitching is due to improve, and it may be, but we'll have to wait and see on that. They'll probably win the division, as they should, but I doubt they go out and win 95+.
Using Adjusted Equivalent Runs, the Nats should have 38 wins and the Braves should have 31. The Braves have benefitted from cluster luck offensively. The Nats have hit .260/.323/.409 and scored 314 runs while the Braves have hit .258/.322/.370 and scored 295 runs. The data suggests the gap between the two in runs scored should be much more significant.
Pitching wise, the Nats have the following:
8.10 k/9
2.38 bb/9
0.72 HR/9
This equates to a 3.25 FIP, which is 3rd in the NL. Using xFIP would rank them 6th in the NL. Furthermore, they lead the NL in pitching WAR. They have been extremely unlucky having the highest BABIP in the NL at .319.
The Braves have the following:
7.14 k/9
3.25 bb/9
1.00 hr/9.
FIP of 4.16 (12th in NL) and xFIP of 4.16 (13th in NL)
Despite this wide game, the Nats have given up 294 runs to the Braves 310.
Some of the problems with the runs given up is due to defense; which fangraphs has as a 10 run differential.
Substituting projected runs scored (using underlying stats to state how many runs the team "should" have scored ) for actual runs scored; indicates a Nats record of 38-33 and a Braves record of 34-37.
I think the Nats have been a bit unlucky pitching wise and the Braves have been a bit lucky hitting wise. Overall, the Nats might be right were their record should be (or a win below) while the Braves are 2-4 wins higher than what they "should" be. The Nats have also had Fister, Stras, Rendon, Werth, Span, and Zimmerman miss significant time.
In terms of projected wins, fangraphs has the Nats at going 52-39 for a 90-72 record and the Braves going 41-50 with a 76-86 record. I think that projection might be a couple wins shy on each team.