Audio Additives Stylus Force Gauge inconstantcy

“Lab quality” my arse.

Aside from the magnetic interference lewm mentions the problem might not be the gauge. A tonearm with bad vertical bearings will do exactly the same thing. Get one of these, https://soundapproach.com/rek-o-kut-roksfg-1-stylus-force-gauge.html?gclid=CjwKCAjwqIiFBhAHEiwANg9sz...

Take repeat readings measuring the distance of the weight side to the platter. If if varies at all you have a problem with your tonearm. There is no way a scale like this can be off unless the weights are bad.
You could also buy another digital gauge and if the readings are consistent the gauge you have now is defective. But, as has been mention the cheap digital scales are a risk. Like Lewm I also use an Ortofon Gauge. I check it once in a while with a calibration weight. I always get consistent readings. The Platform is stainless steel which is not magnetic. 

I forgot to mention, if I were going to buy another gauge I would get the Rega Atlas Gauge. I like the way it locates the stylus in a little cup and it is obviously a much more serious piece than the Chinese gauges looking at the internals. 

@derainer-      I said, "IF" (iow: hypothetical).     That it exists/existed indicates SOMEONE owned one.      Apparently: at some point you assigned a price(s), or- these articles would not have made mention of it:   (https://passionatedj.com/unique-turntables-8-very-different-ways-to-play-lps/)         (https://www.facebook.com/radiotools.ro/photos/with-a-stunning-price-of-650000-the-dereneville-vpm-20...)                                                                                                     (https://thevault.musicarts.com/best-high-end-audio-equipment-to-buy-right-now/                  Don't be mad at me, because you were never able to sell one!

I tried two different scales branded Ortofon and the cheapest is the best! ORTOFON DS-1 is what I like