One interesting angle for me is that I am seldom given complete specs or requirements when asked for an estimate. Of course you ask questions to try to determine key information that has not been specified but often the answers are not available or fully reliable.

So any estimate has to include uncertainty about _the scope of the work itself_ as well as the uncertainties involved in delivering the work.

The natural follow on question when you present a range as the answer to an estimate is: what would help you narrow this range? Sometimes it is "find out this thing about the state of the world" (how long will external team take to do their bit) but sometimes it is "provide better specs".