Time until delivery for good managers and customers is a range. Can you estimate getting 10kg of potatoes from grocery store that is 35m driving roundtrip away? Can you say it will be exactly 40mins because you can pick up and pay in 5 mins? I don't, I can say it will take between 40mins and 2h. There are always things like card terminal stops working or you get stuck in traffic because of an accident.
Complexity in that example is uncertainty like I do expect high traffic and there might be an accident happening but if there is less traffic and I hit all green lights 40mins going to be easy.
We all know bad managers and bad customers will expect me to get that bag of potatoes in 37 minutes and then ask 10x why did I not drove over that police officer that was stopping the traffic because of an accident to get their potatoes on time.