Right, clearly the backend system has different lists, it's simply that DL choses to display all three mixed together on the website/app/GIDS.
They might not actually have different lists. My guess is they use a legacy standby list function and just have a fairly complex priority system that makes sure the list is ordered properly. Remember that delta’s (and most other airlines’) “backend” system is from the 1970s and the primary functions such a reservation records and ticketing still have to fit into this system.