The walking example is really good, I also regularly underestimate how long it will take me to arrive to meet a friend walking, by public transport, whatever.
The reason it's an underestimate even when I know the distances and usual times is because I think about the ideal set of conditions (not missing train connections, clear streets for fast walking, being able to find the place we're meeting instantly on arrival rather than looking around for the entrance for 5 mins) and go with that. I never account for the possibility of missing a train connection by 15 seconds and then having to delay the journey by 30 mins. Even though I know, of course, there is a non trivial chance of that happening.
The reason it's an underestimate even when I know the distances and usual times is because I think about the ideal set of conditions (not missing train connections, clear streets for fast walking, being able to find the place we're meeting instantly on arrival rather than looking around for the entrance for 5 mins) and go with that. I never account for the possibility of missing a train connection by 15 seconds and then having to delay the journey by 30 mins. Even though I know, of course, there is a non trivial chance of that happening.