Look at this AD536, True RMS Converter, salvaged from my first DIY kit 4 1/2 digit DMM, built when I just left school in 1980, obviously.

These chips really square the voltage input, average, and

~~draw~~ take the root, so it's a real analog calculator for the integral of the input signal.

Geeze, that's been 36 years already! I'm vintage also..

True RMS DMMs could be recognized by a big "TRMS" sign on them, so yours obviously measures averaged AC volt only.

(The ICL 7106 is the 3 1/2 digits A/D converter and display driver only)

When I worked in this Airforce calibration lab, also around 1980, we had many of these Fluke 6 1/2 digits state-of-the-art DMMs, I think, already the Fluke 8506A with their proprietary thermal RMS converter inside.

Frank

PS: To answer your question, even if your DMM only measures the average, and will not give the correct absolute RMS value, it will anyhow display any changes of the line voltage.

Maybe your µP TRMS device is not stable; I would test that with a stable sinus signal, as higher harmonics may make a difference.