Sense wire for alternator

The regulator controls the field current to regulate alternator output voltage. The IGN terminal as you know is the sense point. The regulator nominally attempts to keep IGN at 13.8V, but it will be slightly higher when temperatures are cold, to more correctly fit the charge requirement of battery. The alternator is electrically closer to battery, and voltage drop between battery and IGN increases battery voltage by that undesirable amount. The drops in the path typically include battery to starter relay stud, fusible link, bulkhead terminal, ignition switch, bulkhead terminal, IGN circuit. You can easily measure the drop, voltmeter IGN to battery terminal. Fix drops, eliminate regulator over voltage.