The difference between memorizing cause-effect lists and understanding them is what separates a parts changer from a technician. But I agree with your wife anyway, as I also have that same flaw- kills cats, but not us.

Guess:

When the intermittent in your brushes caused the idiot lights to flicker your charging current was undoubtedly varying as well, but the lights could respond much faster to the voltage on the D-61 wire than could the voltmeter, whose movement is damped, could respond to the smaller changes on the main alternator output lead, smoothed by the storage action of the battery. The D-61 wire is the small red wire from the alternator whose voltage is developed from three smaller diodes, separate from the battery circuit, connected to the stator windings.

If your intermittent was much slower, say your idiot lights came on solid for a second or better and went out, you might have seen the voltmeter respond.
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Art Benstein near Baltimore