Simple: We were on a hot streak at the end of August. Now, we're not. Our pitching dominated them for the first two games of that series and we were putting up double digit runs.
It's the exact same team. Whether you want to believe it or not. It's the exact same team. Will it matter, who knows, that's why they play the games. Every team goes through ups and downs in the season.
But the team that was winning games in August is the exact same team that was winning them in September, only healthier.
Besides, the Nats have already won a game against the Cardinals, in St. Louis, in this series - it's not like it can't be done