Based on your description, I too, am skeptical. What is it measuring in the current? Resistance? Voltage? Pulse timing? Something else? I've never seen one of these things before, so I don't know how it's supposed to work.

I could imagine that measuring how well electric current travels up one leg and down the other can measure certain things, but I really doubt that body fat percentage is truly one of them. For instance, I would guess that the current would travel better if you were better hydrated, and that's something which can vary greatly. Maybe a difference in hydration accounts for the different readings you're getting.

I could also imagine it reading different values for people of different heights, or for people of different leg-to-height ratios. For that matter, does it attempt to control whether your thighs are touching? Someone with a space between their thighs would probably read differently than someone whose thighs were touching. That might be a fun test: Try deliberately standing in different positions, either with your knees touching, or your knees far apart, to see if that alters the reading. See if being wet reads differently than being dry, that sort of thing.