Our scales use the Bioelectrical impedance analysis (BIA) method. It sends a tiny current allowing us to measure your impedance which can then be used to calculate an estimate of total body water (TBW). Total body water (TBW) can be used to estimate your lean body mass. By subtracting your lean mass from your body weight it can also estimate your body fat.
Fat mass + Bone mass + Muscle mass = 100% of the body composition
As there is water in muscle and fat, the water mass cannot be added to the other metrics when calculating the body composition.