I believe that your initial premise is wrong. Just because a candidate wins a MOC nomination slate, doesn't mean that they will receive an offer of Appointment.
There are (approximately) 1,500 to 2,000 candidates every year - who are 3Q with a nomination in hand - who do not ultimately receive an Offer of Appointment.
I don't believe that I have seen any data on slate-winner odds of receiving an offer, but having a nomination is only a part of what it takes to get one.
in case anyone is confused, a 'slate-winner' is not just someone who received a nomination. It is someone who received a nomination and is considered by Admissions to be the top candidate on that slate. So, the reality is that nearly all slate winners will obtain an appointment. The ones that don't are usually medically DQ'd without a waiver. It is also possible that the top ranked candidate on a slate might not gain an appointment, that can happen if an MOC issues a Principal Nomination to a candidate who isn't the top candidate - but in that case, the candidate with the Principal Nom would be the slate winner . In those cases, the top candidate might get an appointment via another slate, or could be one of the national top 150, but that's not guaranteed.