> If a plane takes less time to fly somewhere some of the time because of weather, timely takeoff from departure airport, landing slot availability at the arrival airport or whatever, I don't really care. If they arrive late however, that can be a huge problem, particularly for making connections.

This is exactly how developers need to think about how they communicate schedules to management. Missing an agreed upon date is usually a much bigger deal than pushing the agreed upon date out from the get go.