Just to clear things up, this is what I think I understand now:

The King-Sealy (Ford Stock) principle sending units supply a full 6V through the heater wire in the gauge in the form of a square wave with the duration of the "on" being the variable determining the gauge reading. The longer the duration, the more fuel in the tank. It is this full 6v of current which heats the bi-metallic strip in the stock gauge that causes the gauge to read via a small gear train (that may adjustable?). Most after market sending units are simply a variable resistor which sends a constant voltage to the gauge, lessening that voltage as the fuel level goes down. Therefore, the sending units signal doesn't heat the bimetallic strip in the gauge as much. Unfortunately, the calibration of the after-market sending units and the stock gauges don't match up very well due to the differences in the principle of the signal.