Can't change calibration??? Umm by how much are you referring to?

Calibration can change on a mic or a caliper by nothing more then feel.

To heavy handed of a grip will change the reading substantially.

Wear and tear will cause a zero shift. How clean the tool is kept will also cause a failure to zero. Temperature swings will cause mics to read of scale.

Your tool's zero is not fixed and can change. Cleaning the anvils before each use and checking zero is a good practice. But how many of you check through out the scale. Say check it at zero then .250" .5" .750" and even going so far as to check the vernier scale by stacking up enough Joe blocks to get a .0001" shift.
Like .7501" and seeing if the mic with proper feel will hit it. This will show signs of backlash in the screw that can be adjusted out.