Bill,
We use mostly FCAW and SAW; almost no SMAW.
Typically our ammeters on the power supplies agree within +/- 10 amps with a calibrated clamp meter for both FCAW and SAW.
As you probably know, the amperage can be measured anywhere in the circuit; it doesn't have to be near the arc.
Our FCAW wire feeder digital readouts typically read 1 to 2 volts higher than what the calibrated meter indicates. I have not checked the power supply voltage readings when doing those calibrations.
Our SAW tractor volt meters and the power supply meters read the same as the voltmeter as the calibrated meter reads, at least as far as I can determine. The SAW meter is analog and the cal. meter is digital.
When using SAW we use double 4/0 cables for the ground and electrode cables and keep the lengths to 50' max as much as possible. At ~800 amps we get a 3 volt drop between the power supply voltmeter and the tractor meter. If the voltage drop climbs to more than that, I know that I will find a poor connection somewhere that will soon burn out if not fixed.
Our power supply and tractor ammeters read the same.
You will have some voltage drop between the power supply and the electrode while welding. Your 21.5/ 19.9 is really pretty normal.
Picture a long garden hose for watering your lawn. The water pressure (similar to voltage) will be higher at the faucet than at the nozzle. Friction, fitting constrictions, hose length and diameter, etc.(similar to resistance) all act to reduce the pressure available at the nozzle. The flow or gallons per minute (similar to amperage) is the same anywhere in that part of the piping system. Assuming no change in available system pressure, you can push only the amount of water that the hose will allow. Hooking up to a bigger pipe won't increase flow, so you could hook up the water meter anywhere in that system and get the same reading.
I don't know if all that helped with what you're looking for.
Chet Guilford