I think Arya has more powers than we know about at this time. She not only survives the stabbing, but when she talks to Sansa, Arya says that she could not have survived what Sansa what Sansa survived. Sansa replies very slyly that she believes Arya would have survived as well. Sansa also tells Arya that she is the strongest person she knows.
Maybe Sansa knows something that the viewer doesn't know, or maybe Sansa knows something that even Arya does not know yet. Maybe every time she kills someone on her list Arya gains a little more power and strength.
