If I analyze the way they did this correctly, we will receive letters starting thurs/friday this week. Last couple years, UCSF sent out decision letters approximately 10 days after the last interview day, which meant that the adcom met on a monday and tuesday on the 2nd week after the last interview week.
For example, last year, the last interview date was Feb 15th, and the first patch of letters were postmarked on Tues, Feb 26th and Weds, Feb 27th. This indicated that they met on Monday, Feb 25th and Tues Feb 26th (hence the letters were postmarked on 26th and 27th respectively). Also, I believe the adcome only meet twice each week, on each Monday and Tuesday, and decision letters will also be sent out twice each week on the following Tues/Weds.
Moreover, the way they sent out both acceptance/rejection independently of the interview dates during the last couple years showed that they used some sort of ranking system. This is just my speculation, I don't know for sure but this fits the pattern. After ranking each applicant, they will decide to send out acceptance letters to the first few strongest applicants and rejection letters to the few weakest one at the same time. Then, next week, they continue to send out next "wave" of acceptance/rejection letters based on how strong/weak the applicants are. The list will shrink from both end, the strongest end and the weakest end. The one who are in the middle will receive letters last and the rest will be on waitlist.
I think this explains how people receive both acceptance/rejection letters at the same time regardless when they were interviewed.