We don't have any public information about what priorities are used to rank GUC requests, but I strongly suspect fare class does matter, given that Delta has long used that as a tiebreaker for domestic upgrade requests (and the complimentary domestic upgrade tiebreaking priorities are now published on the Delta site).
But in any case moving from 1 to 2 to 3 doesn't necessarily have to do with what tiebreakers are used -- rather, you only show up on the app/website/airport upgrade list once you check in. So if some people hadn't checked in yet, they wouldn't have shown up yet, and appearing to be
#1 on the list 23 hours in advance is a false sense of security. (With the app now auto-checking people in, this longstanding false sense of security may be coming to be less prominent than it used to be, though. But in some cases, especially for international flights, the auto check in does not always fully work, so some people still end up checking in later.)
If the tiebreaker is fare class or something else besides time of request, dropping places on the list could also be due to people changing flights, newly booking the flight, etc.
FYI AA used time of request as the primary tiebreaker for systemwide upgrades for a long time, so also possible you're thinking of them.