calipers can be "tuned" by how they are used. some have hands that can cause variations just because of how the calipers are held. also, remember this. measuring tools are their most accurate when used at the temperature they were calibrated at. this also includes humidity. so now we need to get into the lab with your part and gauges/measuring devices, wait until both have "normalized", then measure. suuure we will....
my point is, you are getting reference information to allow you to make the best decision possible to use in your particular endeavor. this also apples to engine building as well as spacecraft work, although the work conditions are vastly different between the two.