Dean, that's true (the variety of people using the tools) but the second half of your comment stating .001 is .001, not so much.
Yes, on it's face, you are correct - .001 is .001. The National Institute of Standards & Technology defines what .001 is. Fine. What most people don't understand as it relates to "size"; it sounds silly but "size" generally means nothing. Geometry is the key to measuring true functionality. Here's a photo showing several measuring tools that can obtain inside measurements.
I take my measurements using the green "Federal" bore gage. I position the gage & take my measurements north to south as the barrels are positioned proper. I can obtain a measurement out to 1/100,000th of an inch +/- with that gage. (splitting the graduations of 1/10,000th by eye). That's literally an inch broken into 100,000 equal segments for goodness sakes! Pretty precise? Not so fast! Drop the Comtor gage shown in the photo into that bore and compare that measurement to what the Federal gage says. It'll be smaller. Why? Because the Comtor gage is taking every variance of the bore such as an out of round condition, et al into consideration. The Comtor gage is telling you the largest size of a perfectly round shaft that will pass through the bore..THAT is true and bonafide accuracy! The Mahr bore gauge would be the second most accurate way to obtain a true measurement.
The Mitutoyo dial calipers, I suspect, are an example of what most people are using to obtain measurements. Trust me, that's not true precision but, it's close enough. The Federal gage I use, measures the bore in the same manner as the dial calipers however, it's much, much more accurate and has the additional advantage of detecting variances and geometric shapes in a manner the calipers simply cannot.
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