THD math help please

I am trying to reconcile the difference in Total Harmonic Distortion readings I get when using an Amprobe ACD-41PQ meter compared with the number on my Power Plant P20.

The Amprobe ACD-41PQ meter is a handheld, clamp style, meter that lists the THD on screen. I get readings between 27% and 77% at my house. In particular, if I measure the line my P20 is on, I get 77% on the Amprobe. However, the P20 reads 2.7%. I think there is a math calculation to reconcile the difference, as I don’t think they are presenting the same exact information.

According to the Amprobe manual:

"THD%-F Total Harmonic Distortion-Fundamental function (model ACD-41PQ only)

THD%-F = (Total Harmonics RMS / Fundamental RMS) x 100%

Total Harmonic Distortion - Fundamental (THD%-F) is the percentage ratio of the Total
Harmonics RMS value to the Fundamental RMS value of a voltage or current signal, and
is given by the above expression. An ideal sinusoidal waveform has a value of 0 THD%.
A badly distorted sinusoidal waveform may have a much higher THD% value of up to
several hundreds.

When the meter is in ACV or ACA function, THD%-F values up to 99 THD% will be
displayed in the secondary mini display automatically. Press THD%-F button
momentarily toggles THD% readings to main display to get full readings up to 999.9
THD%. "

Does anyone understand this? Can you tell me if there is a way to convert the Amprobe reading to the reading I see on my P20? I want to use it for comparison purposes, however the 77% number on my Amprobe meter is meaningless to me.