I believe I understand how the altimeter works which is as you have described here.
In the context of this conversation, which I set the assumption that the units are on the ground and not moving, you indicated that the static pressure still plays a part.
Considering the static pressure will be the same in the cabin or outside of the cabin AND will be the same for both units, how would this effect one device differently than the other.
I did not say it would have a different effect on one vs the other (different effect no but what they read yes...more about that below). What I said was static pressure does have an effect. You said it had nothing to do with it which it does.
This conversation has nothing to do with static position error or the effect of leaks in the system or the difference between the pressure outside the plane vs the inside while in flight.
Since static pressure is half of the equation in how an altimeter determines altitude, I say static pressure is very much involved. I do agree that on the ground it will be involved the same on 1 or 10 altimeters (what they read is a different story...see below).
The calibration simply determines what this static pressure is referenced to and in some cases how linear it is across the span. The Garmin AD-AHRS units have the ability to calibrate to the zero and a minimum of 2 and optionally 3 other points. This improves the accuracy across the entire span.
That being said, since there can be error in the calibration at one point where there is none at another point and this could be different on different altimeters, the static pressure at different field elevations could definitely cause a different "reading" on different altimeters attached to the same static system. The same airplane may not show any difference at sea level on a standard day but might be off by some amount at 5000ft or any other point in the span. Or they could be dead on at 1300ft but off at sea level on a standard day. Or dead on from sea level to 10,000 but start to drift apart at higher altitudes.
Example: My airplane may have zero difference between the altimeters while on the ground at my home base which is 1313ft of elevation. It may have 20ft of difference at El Paso which is 3958ft of elevation with the exact same atmospheric conditions. The only thing that changed is the static pressure because of the increase in field elevation.
I also agree that the only way to know which one is off is to use a calibrated test set with enough resolution to act as a certified reference. Like Levi warns above, don't mess with it if the difference is within tolerance... You could make it worse!