It is isn't that hard.
Pitching was a bit below average. Scherzer was awesome, everything around him in starting rotation was a disaster. In the bullpen, the Nats had three closers go down with injuries. Defensively, Nats were excellent about catching the ball when they got there, but that just hides the fact they had a below average overall metrics. Any outfield combination that didn't include Taylor or Stevenson was ugly.
The offense was pretty good but horribly inconsistent as been outlined multiple times over.
Below average pitching, below average defense, solid but inconsistent bats is a pretty easy formula for 82-80.