Anyone Running A Mechanical Voltage Regulator 67 Dart 273 2bbl

As far as I can tell, the solid state regulators do the same thing, but perhaps react faster.

I think Chrysler's description from 1959 best desribes the cause of the meter fluctuations at idle.
The available alternator power finds the easiest and shortest path to the alternator. So it goes to the ignition and brakes lights first, then what remains is available for recharging the battery.
At idle speed, it doesn't take much load to cause the voltage to drop and then there are two power sources with the same voltage. We see the battery discharge as soon as its voltage and the alternator's are equal. The ammeter fluctuates as its just going over the line between discharging and charging.

I think the regulator's own draw, and the rotor draws are contributors. Some of the most noticible fluctuation I've seen are with revised squareback alternators. Also in hindsight, it seems my electronic reg failures have happened mostly with the squareback and revised squareback alternators. Their rotors seem to have higher power demands.

One day it would be interesting to run a points regulator with the cover off and see how quickly (or slowly) the points move between the positions.