[adisinom]

"You would expect that if we were getting it wrong, we would be getting it wrong the other way, delivering early."

How likely is it that a surprise works in your favor? In my experience what you don't know rarely does. So my mental model is that completion times aren't normally distributed; they have a long tail. Additionally a task can't take negative time. This makes estimation tricky because a task can take much longer than the average. At my company we typically report an average and then explain why things took longer.