The misconception seems to be about what the voltage regulator does.
It's only job is to try to keep the alternator output voltage constant,
no matter the load on the alternator. It does not know or sense what the
charge level is on the battery. It just tries to make sure that the
alternator is keeping up with demand but not exceeding it. That demand
can come from charging batteries and/or running headlights, fans, air
conditioning, whatever. If the demand goes up, the voltage regulator
causes the alternator to supply more current to keep the voltage from
dropping due to the additional load. The voltage regulator does not
"kick in". It is ALWAYS working, trying the keep the output voltage of
the alternator at a constant voltage of around 14 volts. The alternator
can put out a much higher voltage than that, if the regulator didn't
stop it from doing so. That is why it is called a REGULATOR. It is
always keeping the alternator in check, to prevent the voltage from
being too high. That is it's main job. Too high of a voltage will damage
the battery and possibly other electrical stuff.

Mark