But you said that "I don’t think any of your conditions have to be true for estimates to be accurate" and one of my conditions was that the requirements should

But you said that "I don’t think any of your conditions have to be true for estimates to be accurate" and one of my conditions was that the requirements should be correct and shouldn't change. And the point I was trying to make was that - in my experience - is that often requirements changes that are a huge driver for estimates going wrong. If you're lucky, you work in an environment where a changed requirement automatically results in re-estimation at the customer's cost. But in the places I work, the requirements are often not well stated in the first place, so they are subject to interpretation, and it is very difficult to estimate reliably in such an environment. And for whatever it's worth, that's been my experience for the whole of my long career. As engineers we're forced to quote against crappy requirements, and as management we're forced to accept crappy requirements in order to win the gig. But all too often engineering is blamed for late delivery of something that sales shouldn't have sold in the first place.