Originally Posted by remote_goddess;2499091;
It's probably a matter of intent. In the last book Harry performed the Cruciatus curse on Amycus. It worked well and he told McGonagall that Bellatrix was right that you really had to mean it for the spell to work fully. The same holds true, I would think, with expelliarmus. In the practice sessions, the DA members wanted to get their partner's wands, yes, but they didn't want to get them in order to use the wand against the other person or for any other negative reason. Nor was it a case of them really battling the other person for the wand - they were just practicing. As such, they didn't really "mean" to disarm their practice partner, and the wands recognized that. That's my conjecture, at least.