I actually wrote 'ar urbem non quirimus' because that was what I heard. I knew the first and last words were not correct, but could not decipher them. It might have helped had i known she was saying 'nunc' rather than 'non'. Had others not had problems, i might have just put it down to my speakers in conjunction with her rolling Rs.
They probably let you pass with a typo, which the system sometimes doesn't report to you. Up until a few months ago, any one-letter difference would be considered a typo; now, generally if the typo is such that it turns into another word which Duo also knows about it will mark you wrong, though sometimes it will do the same for other, unknown, reasons.