I always thought that his hand was hurt during the fighting with the Sranc. And, I agree with Simas, they thought him defective for some other reason. I could be wrong about the hand, but I don't remember it ever being stated as that was the reason for him being defective.
I am very excited to read about him, and his POV of the world as an untrained Dûnyain. I think it might be the case that he is on a higher level than Koringhus and that they never noticed it because he was thought defective. It's great that he's basically superhuman, but doesn't have the outlook on life like other Dûnyain, the Logos and all that.