These scales essentially just run an electric current through you and measure the resistance don't they? Maybe some passive electrical too.
I've wondered how much moist feet one day versus dry feet another day change readings. However even though I have a scale that does similar stuff, I tired of using those functions long ago and never got interested enough again to try out the moist vs dry feet thing. I just use the scale to weigh.
Maybe even one moist and one dry to throw another twist at it?
Understand I'm not discounting how accurate they may be, but like a lot of other things they are relying on generalizations from indirect measurements.