[ChrisBennet]

My thoughts are along the lines of 304065's (measurement process).

I measure deck height to flat band around the circumference of the piston face (what I call the "ant racetrack") using a tool I had made. Since the piston can and does tilt in the bore, I measure at 4 places (in line with the pin axis and at right angles to the pin). That gives me the worst case value and an average deck height for the piston.

A possible source of error would be tilt of the cylinder. Are the cylinders held down with 2 cylinder hold down "nuts" each so they are flat?

Use 6 hold down nuts to hold 3 cylinders down on one side (2 nuts each). Then place a machinists flat edge or piece of ground stock across the top and check for flatness.

If you measurement technique is valid, your measurements will be consistent i.e. if you measured those pistons again, you would get the same values. If your measurements are inconsistent, you simply can't trust your numbers.