It'll vary by the school but some places definitely treat URM as an entirely separate pool that is only in competition with itself, and your odds of an A or getting pulled from waitlist will all be relative to the number of other URMs they prefer to you or that turn down their offer in that year.
As an example, the year I applied, there was a URM applicant that got waitlisted. Later on in spring the URM applicant not only got admitted but offered a large recruitment scholarship. So being URM didn't mean they were a shoo in admit, but kept them in a small separate group that was extremely advantageous once the school saw most of their admitted URMs were matriculating elsewhere. So being URM can still be very influential on your final outcome after interviews but it's not always so straightforward as higher post II admit rate.