Sorry!, - Your logic tells you the wrong, also as we don’t know how low your n is.
On the other hand as long as the values for the RSD are low, as yours, it may hold as a first approximation and you have not too much to worry about.
For the explanations you have to keep in mind:
- that SD is the square root (sqrt) of the variance, and only variances are additive.
- that in real we should correct SD/RSD for low n by the “Student-Factor”
For your calibration you did two observations, the concentration and the mVolts. And as I’m quite sure you declared your concentrations as errorless, so you incorporated the variance into the variance of your mVolts and so your SDcalibr contains both errors.
But for concentration estimations you do always a third observation, the one for mVolts, that means that your total variance will increase by a third term, but will not be doubled as explained above.
As the SD/RSD is the sqrt, the situation looks even less dramatic, and that’s why for low SD’s you have not very much to worry about.
More dramatically would be, if we correct your RSD by the “Student-Factor” for low n’s.
I hope to have been of some help to you.