Schedules are tough for a transit system to keep. Plus you aren't guaranteed to have the same walk to the station every day. Factor in maybe a couple light cycles of signals you need to wait for as a pedestrian to get down to the station, and your travel time on foot to the station can vary by a significant proportion, adding one or a couple three minute light cycles into the mix that might or might not hit for you. I try and anticipate getting to the platform at least 10 minutes before the scheduled train as a result. If I am closer to 5 minutes then I am rushing, feeling late, and stressed, and have missed plenty of trains in moments in this situation before.