^^I've tried to explain this^^ It's called voltage drop. The pathway (might be slightly different on a pickup) is from the battery---to starter relay "big stud" (junction point) ---------through the fuse link-------to the large red ammeter wire through the bulkhead connector--------to the ammeter--------through the ammeter-----out the ammeter on large black--------to the welded splice---------branch off--------to the ignition switch connector-------through the switch------out the switch connector on "ignition run" IGN1 wire---------back out through the bulkhead connector............into the engine bay, and branch off to feed underhood loads, including the voltage regulator IGN terminal.
EVERY POINT between the dots is a potential for a poor connection and some voltage loss. The bulkhead connector, the ignition switch connector, and the contacts inside the ignition switch are likely the worst offenders.
ANY VOLTAGE loss through that pathway results in low voltage at the VR in comparison to the battery----just like you measured. The voltage regulator tries to maintain nominal 14V AT IT'S IGN TERMINAL. That means when running, the VR is trying to maintain that, and the drop now works "in reverse" so to speak, as now, the battery must be HIGH to maintain the VR IGN terminal at 14. So the overvoltage condition at the battery when charging is equal to the amount of the drop you measured.
The drop is additive, meaning there can be a few tenths in the ignition switch, a few tenths in the bulkhead connector terminals, and so on. The total drop is the amount of overcharge voltage. ANY drop in the grounding circuit is also added to this drop
