The regulator's sensing is usually taken directly from the
alternator's output terminal, not the bus, and voltage variations at
the bus may not be present at the alternator's sense terminal so it
might not respond to a seemingly low system voltage. This could happen
if there's a bad connection between the alternator output and bus, such
as the crimped connections in the output cable, connections at the
alternator output breaker, or the breaker itself. None of these are
unusual, and the voltage will usually drop further as load is applied
and the resistance in the connections makes itself known.
That said, the regulator is often overlooked in the search for
alternator problems. The regulator controls the field current in the
alternator, and if it's not doing it well the output voltage will be
off or erratic. Lots of alternators have been replaced for no reason
other than a bad or misadjusted regulator.

Dan