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In 50 years of reloading I have never had a bullet measure other than printed on the box!
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That is not surprising. It is due to the sensitivity of the measuring instrument. In your case, a micrometer.
In measurement science, there is a hierarchy of standards. The highest order of standards are those that are used to transfer their value, and the associated uncertainties, to lower order transfer standards. Those lower order transfer standards and THEIR uncertainties are used to calibrate working standards. Those standards and their uncertainties are used to calibrate things like micrometers. In every case, a higher order standard is used to transfer/calibrate a lower order standard. In every case, working standards can never be more accurate than the standards used to transfer their value NOR may they be used to calibrate in reverse order a higher echelon standard.
Another way of saying it is you may use a very precise cesium standard to calibrate Big Ben. You cannot then assign a value to the cesium standard by validating it against Big Ben.
The continuity and hierarchy of standards is what allows interchangeability of parts from different manufacturers.
Your micrometer was probably originally calibrated to +/- .0002 inch by a gageblock set whose accuracy was on the order of a few millionths of an inch.
That gage block set in turn had the values assigned to it by a transfer standard and that transfer standard had its value assigned to it by a master.
The masters are often calibrated by NIST and those standards derive from a natural physical constant such as the wave length of a particular helium neon laser.
Failure to abide by the rules will almost always lead to a departure of the desired attributes of a produced part when comparing what you have in your hand to what is defined in the part specification drawing.
It is much like a band tuning up when the guitar player is tuned to a standard concert pitch A=445, the bass player is tuned to concert pitch A=435 and the piano player is tuned to A=440 standard. Everyone is in tune but the music sounds like crap.
The higher you go in calibrating standards, the more complex it is.