71 Dart overcharging

Just for curiosity, why would a .2 volt to .3 drop make any difference when not running? The regulator comes into play when the engine is running, therefore the voltage goes up, and the voltage becomes the same at battery as at the regulator. I had to think about that one.
There is always going to be a drop when not running through the ign. switch, because of things like the resistance from the switch, wiring, connections, guges etc. Especially when the batt. is in the trunk.
I did try a wire from the batt. cable, to the regulator, which made the voltage the same at the regulator as the battery 12.5 not running. Made no difference in the output of the alt. when running
It's all good now, it is charging 13.9 at above idle max. The .8 resistor was the issue. It had been sitting after I thought about it. The battery gets down because it has a memory stereo. It also has a Holley with an electric choke. Once it ran awhile the voltage came down. I also discovered the aftermarket gauge is about .3 higher then the real voltage with my Fluke.
Thanks for all the input
Chris