Voltmeters and ammeters that do not directly measure the true rms value measure the peak value and divide by sqrt(2) to show the rms value. This is the true rms value only for a perfect sinusoid.
The almost universal paradigm for the pre-digital analog meters was to rectify the measured voltage (voltage across a load resistor for ammeters) and then measure the average voltage. This was scaled by the "form factor" (1.1) to convert to RMS.
As jghrist says,
"Only valid for a sine wave."
I had the misfortune to use one of the early Peak measuring digital ammeters. A disaster. Don't forget that harmonics and surges have peaks also. Often with little relationship to the peak of the fundamental.
I suspect that any multimeters that do not report true RMS will be using the old technique of measuring the average rectified DC value and scaling this to RMS. I have seen a couple of examples of issues with recognizably erroneous readings with distorted wave forms, but by and large the technique produced usable results. (For decades).
Peak scaled readings not so.
Bill
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"Why not the best?"
Jimmy Carter
