"Film-based photography suffers the same problem,..."

Yes, every few hours in the lab we would do another calibration (grey cards with different exposures - and measure them on a densitometer and input the changes into the machines) - every other change of a roll of paper, and every few times we have to change chemicals. It usually wasn't that big of a difference, but if we did NOT do the calibration at least once a DAY it would go haywire very quickly - things will start coming out more and more off color and density - it would be a nightmare to adjust for.

The short tidbit to understand - when you go to ANY chemical lab, you will NEVER get the exact same print twice. It may look really close, or unnoticeable at a casual glance, but look closely and subtle shading will be different, shadows might be a little greener, highlights might be a bit more magenta, it might not be as contrasty, or MORE contrasty. As soon as 20 minutes after making a print, the chemicals have changed enough to not make a truly exact copy.

So many picky customers leave a lab thinking the tech have NO idea what they're doing - they just don;t understand there's about a 1,000 variables that go into making the same print twice, and it'll never happen.
-Lew ;-)