I heard a great interview of a Tesla engineer speaking about the “nines.” I could not find a transcript of the interview but this link contains a great summary:
I think basically what takes the long amount of time and the way to think about it is that it’s a march of nines and every single nine is a constant amount of work, so every single nine is the same amount of work, so when you get a demo and something works 90% of the time, that’s just the first nine, and then you need the second nine, and third nine, fourth nine, fifth nine, and while I was at Tesla for five years or so, i think we went through maybe three nines or two nines. I don’t know, but like multiple nines of iteration, there’s still more nines to go, and so that’s why these things take so long […]”