How could you know that your measurement result is 100% accurate?
Is that because you use an accurate measuring instrument? Not that easy.
What if there are many things that you are missing; things that you have to take into account. External and internal sources such as temperature, humidity, heat thermal coefficient, operator error (parallax error, poor eyesight, etc), cosine error, abbe error, random error, measurement method, and internal measuring system are just some factors that impact the measurement accuracy.
Your micrometer gives a reading of 1 inch but it can be actually not. It can be 1.001″. The extra 0.001″ can be generated from the material expansion of the micrometer. Your way of measuring, the room temperature, and your hand temperature can influence the heat to the micrometer. Then the micrometer material (stainless steel) expands. It’s not weird that the micrometer you are buying gives information that its accuracy is ±0.001″. This information indicates that whenever you take a measurement reading, the true reading lies between the -0.001″ and +0.001″ addition.
Measurement uncertainty is a vital thing in calibration. When calibration is performed, the uncertainty is measured. As a thumb of rule, the lower accurate that standard you are using, the higher the certainty.