Mathew, as this is such an " in depth " discussion, have you considered the difference between the legally adopted British inch and the inch adopted by the USA.?
The 1922 comparison gave the British inch as being slightly less than 25.4mm and the U.S inch as being slightly more than 25.4mm.
Technically the U.S.is metricated in that its measurements are conversions of the metric SI system.
Apparently there has been between 5 and 8 different inches throughout the history of our planet.
And yes! I am just sh_ _ stirring.
The difference between the two above inches is 1 ten thousanth of an inch over 27 inches. (zip!)