Did the imperial army just get used to fighting the Swedish armies in between it's introduction and the time of Nordlingen? Did Gustavus Adolphus' leadership matter that much? Even then, what had changed between then, and the later Swedish victories in the 30 Year's War?
Is there any reason what so ever to assume there was a significant difference in the Swedish army between the two battles? The defeat at Nördlingen happened primarily because the cavalry was uncharacteristically undisciplined, the regimental artillery was left behind and the Swedes ("Swedes") were significantly outnumbered. Given the preformance of the Swedish army in subsequent battles such as Wittstock, Breitenfeld 2, Jankau etc I see no reason to attempt to explain the loss at Nördlingen with a difference in the army. The Swedish army wasn't unbeatable at Breitenfeld and they weren't sure to lose at Nördlingen, it was in most respects the same army.