Many companies have their equipment calibrated on a regular basis and believe all will be well until they are calibrated again. Then the follow-up calibration shows one or two instruments were not able to perform up to expectations. Now what? A worse situation occurs when an instrument fails in service and suspect work is shipped and then discovered by the customer. This can happen because calibration deals with a situation at a fixed point in time. You could reduce this risk by calibrating instruments before and after each time they are used but that is not usually a practical consideration. And the effectiveness of it diminishes in proportion to the time between these two calibrations.
You have to remember that the calibration process deals with the instrument involved, not the measurements it produces. An instrument that passes the follow-up calibration could have been used in measurements that were out to lunch due to other factors. What other factors? Two culprits that come to mind are temperature variations or manipulative variations.