Most sites would refer to Day 0 as the day the person installed the app - wouldn't it be 100%? Can you tell me what you mean by Day 0 and Day 1?

Day 0 is the day on the install, and Day 1 is the following day, and so on.
Branch tracks install as the first open of the app. The second open on the same day is coined as OPEN, which is taken into consideration for the cohort retention report. That is why it does not show as 100%. The other days will be considered the first open on that particular day.