Underperformance is not even the appropriate term for what we're doing... we are scoring enough runs to win, we are just finding ways to lose, like not scoring timely runs, commiting costly erros (even when our fielding is good throughout the game).
Here are some key statistics:
The most glaring one is from Baseball Prospectus and their adjusted standings (which I got from Mike's table). According to those standings we should be in first, and the Nationals in last. We have been incredibly bad at winning these kinds of games, while the Nationals have been incredibly good at it. You may chalk it up to luck, and hopefully luck evens out, but it's been an incredible disparity.
A nice chunk of that disparity between expected Win-Loss and actual Win-Loss has been made up in one-run games, and games won or lost in the last AB.
Our record in one-run games is 7-12. We have lost 6 games in the last AB and won only one in the last AB (Run Joe Run). What's odd about that is that we actually outscoring our opponents 13-4 in the ninth inning, and our bullpen ERA is not terrible.
Again, the problem goes back to scoring. Even though we lead baseball in big (>5R) innings, we have only won 8-21 (0.276 win pct) when our opponent scores more than 4 runs. While this is nothing new to the people on this site, it's dramatic when you see that number. We cannot win when our pitchers give up more than 4 runs in a game, even though we lead the league in scoring >5 runs in an inning? That's just weird.
Luck or bad clutch play? I don't know the answer to that. If I did I would probably be managing the Marlins instead of crying about them. But I do know that we are the antithesis of the Nationals, and the antithesis of what the 2003 Marlins were (in terms of clutch and late-game play, we are similar in terms of Run scoring and pitching).
I hope it's "luck", or the random nature that enters into any game. That tends to even out over time (regression to the mean). If/When it does, we will be a force to reckon with.