My guess is that it’s related to how United elects to post information on the app. There are a number of times that I’ve checked flights and discovered that the aircraft scheduled to perform the flight shows an arrival time at the gate after the posted departure time for the next flight. The information is clearly inaccurate or at least it’s not current; however, there’s usually an equipment swap, which saves the day and gets things back on schedule. Next time I see it happen, I’ll check Flightaware too to see what it says on that site.