That makes sense.
I assume you understand that if the regulator 'sees' a low voltage on the J2B wire, it assumes that is system voltage and lets more power through the rotor.
So if the regulator sees 13.5 V, it thinks the system voltage is low, even if the alternator is producing at 15.1 V.
With the improved connection, the regulator 'sense' or controlling terminal is getting voltage much closer to what the alternator is producing power at.