The purpose of the original test was to test her "movements". I Assume that was her reflexes and style of fighting. By incapacitating her opponents immediately, she ended the spar too quickly to get enough meaningful information (or at least the kind of info the captain was looking for). So the test was inconclusive. Whenever the test itself fails, you start over with a new/different one. I assume that he is either acting for a life or death, against an overly strong opponent, kind of test now. Or he has actuality already made his judgement that she is inexperienced in real combat/ naive/ careless. I personally don't like the latter cus it is prejudice and something that all knights train for when they join, and not an actual reason to turn her away.
Either way, I am on the side that says she will eventually join, for whatever reason, cus the plot/drama demands it.