That ponders the question about how do we know what we are using, and can we trust it.
As we go through this hobby and to be honest whatever we do even outside the hobby there is always deviation in a result.
We all try to take good care of our Corals and Fish, to the best of our abilities. It just gets to be a matter of trust on our part.
We trust the calibration fluid is right, then its not, and no accountability is ever taken or enforced, when someone is in this situation.
Where does it end?
It ends when you buy 3 different brands of calibration fluid.. lol. $$$
I agree with your post. At a certain point you just have to go with something and not end up settting up a lab in your home with tested scales, containers, and who knows what else to verify 100% you are correct.