Someone said they try to fill seats with a person of a similar demographic to the person who gave up their acceptance. I think thats the only way that a non-ranked list could work. It also makes sense why it would take longer because they have to meet to come to a consensus that the new one is the best fit. I imagine that means they have sort of ranked different factors of each applicant but not a ranked master list.
Personally, i was initially a little upset after hearing Pitts waitlist would be non-ranked, but after some reflection i do think its better for making sure the incoming class is more inclusive of people from different SES or non traditional students. Ranking is more likely to favor priviledged and traditional students. Non ranked is more likely to value the different experiences and perspectives that could be contributed to the incoming class