For measurement of small nuts you apply an error calculated for 10cm. If the nut is small, the error will be also small, not 0.5mm, but something like 0.02-0.03. So if you measure 4.0-4.03 you won't make a mistake which nut it is. You can live with that as a DIY-er unless you need to measure 10cm+ nuts with high precision, I doubt that.
No, that's incorrect, because for small measurements the slop as well as the influence of bad geometry is going to be larger. Lack of accuracy is a problem that multiple issues contribute to.
I don't know where you got your numbers from. I doubt it's anything other than you thought the numbers just looked fine and illustrated your point.
