A friend of mine had her body fat % rated at the gym, and asked about the scales (since I said I had one). They said the bathroom scales measure fat biased to your lower body (like someone said, the path of least resistance), while the calipers method they did on her is biased toward your upper body. If you carry more fat in your lower body, that could explain the readings. The only truly accurate way is by displacement, which is a little more difficult to do at home, unless you have a very accurate pool

They work by sending an electrical signal through your body, so I'm assuming the signal will get dampened differently by fat than by muscle or bone, and the scale can measure the difference... or something. It measures your weight first and asks for your height, so it must have some intelligence.

Those Ironman scales that measure everything under the sun... whew!