Double-A Harrisburg clinched a spot in the Eastern League playoffs for the first time since 2002 today, after Bowie lost 4-3 to Richmond. The Senators won the wild card in the Eastern League, and will start the playoffs at Altoona on Wednesday. They are the second Nationals affiliate to qualify for the playoffs this year, with Single-A Potomac having already clinched a spot.
One thought on why you're seeing the Nationals' minor league teams post better records (aside from better talent): there's not as much movement in the minors as there was during GM Jim Bowden's tenure. Because more players are staying at one level for more of the season, teams get better players who have the benefit of time to gel together. That's especially going to benefit teams in the middle of the system, like Potomac and Harrisburg.
Whatever the reason, the Nationals will have two of their top three affiliates in the playoffs this year, with Triple-A Syracuse having posted a winning record, as well. That's got to give the team some encouragement.