This is not a problem. Or it is just a little one. ;-)

I would be grateful if someone could explain why voltage (while running) on my 1987 740 (looking at the analog gauge in the dash) is LOWER if I start my car with the headlight switch already on, and higher by 1/2-1 volt if I start the car with headlights switched off -- and then switch them on.

I mean...
1. headlights switch left on, start car, get shy of 14 volts (more or less)
2. switch off while starting, start car, turn on headlights, get maybe 14-1/2 volts

This is consistent, across seasons.
The regulated voltage does, of course, decline as I add additional loads, such as a HVAC fan, or stereo, and wipers.

The regulator is one of Dave Barton's adjustable units. Internal. (I mounted an external [remote] one on our 240, for easier access.)

I noticed this phenomenon because I tried to get religion, and always run with headlights on. So I eventually started leaving the headlight switch on... and noticed the reduced voltage; which especially mattered as cooler and damp weather came, and fan and wiper loads increased, and voltage fell to punky mid-range (and below mid-point) on the gauge.

Now I guess I am back to doing the lights manually, because I would really like to see 13-1/2 volts to 14+ anyway.

Thank you!

P.S. Is the word "excitation" somewhere in the answer? Dunno about any Good Vibrations in the car, but a little excitation sounds good!