supercuda covered it pretty well.

The sense line (blue wire) goes from the battery, through the bulkhead (twice), through the ignition switch plug (twice), through the ignition switch itself, and maybe another connection or two along the way. Any or all of those can be less than ideal.

My solution was to mount a relay under the hood, have the old blue wire trigger it, and run a new wire through the relay to the alternator/ignition subsystem.

No more overcharging for me.

Edit: if you do find a large voltage drop, you can temporarily install a jumper from the battery + to the blue wire at the alternator to bypass all of the voltage drops. That should immediately bring your charging voltage back down. Note that it will no longer turn off with the key while this jumper is in place