From what I've heard through the grapevine, all candidates are ranked using a numerical system with each category (GPA, LoRs, interview, etc.) weighted differently. My best guess about how the system works is that the AC list is ranked, and candidates who interview in Round 2 will be incorporated into the list based on their stats. The admissions committee will then take the top x number (15? 20?) to fill their quota for the second interview round and so on and so forth until all interviews are done.
If someone on the AC list from Round 1 is in the top x of the AC+Round 2 combined ranking, then they will get accepted after Round 2 interviews. It is possible that a Round 1 AC will not hear a definite "yes" or "no" until the very end of the application cycle in March. You could get accepted anytime between now and then.