The plane makes multiple stops throughout the day, and various things happen on each leg and at each stop and time adjusts accordingly.

Say it left it's first stop 30 mins late, so your time pushes back 30 mins. On the way to stop 2 it makes up 10 minutes in the air so now your time goes forward 10 mins to 20 mins delayed. But at Stop 2 the ground time is 5 mins over and now you are 25 mins delayed. And that's just one leg of many before your flight.

I guess the time changes are significant enough to warrant a notification? I wonder what the threshold is.