how do those hand held things work - they send an electric signal through your body to test resistance or something? does the scale at home do that? do you have to stand with bare feet on sensors or something? maybe foot calluses are affecting the reading.

anyway any 2 scales from different manufacturers would register different weights, so I would assume the same is true of this kind of measurement. so just use one. maybe the home device can be calibrated? can you go to a water tank and get a precise measurement and then calibrate the home device to match?