Just from a physics-based opinion, things that require calibration do tend to drift over time. Component aging, thermal/environmental cycling, physical impact (dropping on the ground), and overvoltage can all cause calibration errors. That's why my company has a calibration lab, and all test equipment is on a calibration/recalibration cycle. How often for the Nano is how confident you are in the saved cal vs the actual cal when it is done again.
