Well, according to conversations with their support their numbers should fall within 5% of actual.
And the explanation for the difference is described within this graph. Whereby a true RMS meter takes the reading while the phase is in sync , and the subject Load Controller (and power meters in general) do not.
I understand the effect power factor (pf) has on the actual power (vs VA). However your stated issue was the current is different. I know RMS current and RMS scaled from average current will be different, however 2 amps out of 6 (or 8 if you like). Is a large difference and the crest factor (a partial measurement of waveform distortion) would have to be severe.
So there is no way for us to know what the difference is. I guess I would think differently if we knew the multimeter brand. Or maybe not.
You're right. It is a large difference. I've looked at enough stuff with this Fieldpiece clamp-on meter to think it's always been in the ballpark of expectations...but maybe it's off. I'll do some more checks but I see the reported wattage as a relative and still useful tool ...just not absolute translation of load.
We'll just let this thread stand as a place for other owners of the RM3250ZB to report their readings vs their load specs/expectations on into the future.