I really cannot understand that you talk about micro meter (um) accuracy on a caliper. If you apply a tiny bit more pressure or it is under a tiny angle, the error is at least tens of um.
EDIT: ah, I see your confusion. A "micrometer" is an aliased term. It means both "a millionth of a meter" and "a tool for measuring very precisely". I used the latter meaning above. Although in terms of order of magnitude precision they're identical--a micrometer accurate to 0.0001" is accurate to 2.54e-6m. It's possible to get within a handful of µm with decent calipers. Easier with a micrometer.
You don't need to flex the material you are measuring to get tens of microns difference. Simply both sides of the caliper will not stay parallel since there is always some dust or fluid between the sliding pieces. The dust will simply flex.
Of course. To get a good reading you have to clean both surfaces. I mean if you want to wring blocks together it won't work without clean surfaces either.
EDIT: ah, I see your confusion. A "micrometer" is an aliased term. It means both "a millionth of a meter" and "a tool for measuring very precisely". I used the latter meaning above. Although in terms of order of magnitude precision they're identical--a micrometer accurate to 0.0001" is accurate to 2.54e-6m. It's possible to get within a handful of µm with decent calipers. Easier with a micrometer.