Cheap scales don't measure body fat uniformly. They ignore arm composition. For the purposes of standarization they give different answers than the expensive devices used in clinical studies.

Fitness studies also measure more than body fat. They measure the circumference of various body regions. I don't think a measurement that doesn't take into account the shape of a body produce a good answer.

why do you think a device two orders of magnitude more expensive would?

Most medical devices that set standards aren't very cheap. Very cheap devices give nobody an incentive to run the studies