There is a simple electronic equation E = I X R (voltage equals Current times Resistance). Resistance can also be the "Load Resistance". If you hold any one variable constant and change one of the others, the third will change to maintain the balance.
Plasma is a constant current cutting process. When you set the current on the dial , the plasma unit will try to maintain that current (I). It's a balance relationship. Since the cut gap (arc length) represents the "load" (resistance), change of that distance (R) changes the other leg of the equation (E) in direct proportion.
In simpler terms the gap voltage is proportional to the gap distance if the current is constant. It only takes about .010 inch of difference to make about 1 volt of difference in the gap voltage. It's like measuring the change in air pressure to determine altitude.
Each plasma manufacturer has determined their specific voltage at a given cut gap. For some it's higher or lower than others but it will remain relatively the same for that given machine. The charts in the owners manual are based on a given torch, nozzle and cut current. If you use a value outside the chart (lower/greater current on a given tip) you find the voltage at a given gap is different.
Plasma cutting air has the highest voltage. A tip sitting on the metal has the lowest. measure the gap voltage, eliminate the noise, and be able to move over the change of just a few volts (out of hundreds) and you can "servo" the Z to move the torch tip small amounts to make maintain cut gaps that have to stay within + - .020 while moving in X & Y at 200 IPM.