Lady,
I had an appointment at Curves last Monday and she made me grip on a machine that measures your body fat. (Horror! 30% according to that machine). She said the machine sends some electric current through your arms and determines the body fat. I believe the Tanita scale asks you to be barefoot to determine the body fat and to place your feet at a certain place (I bought mine but it was not working right so I exchanged it and the new one is still in the box). So maybe the scale uses the same system. This is just a guess, no scientific knowledge here. I believe the books use a porcentage system based on height and weight. They could not use any other system since they don't have you in front of them to determine that!
If you go to a health club you may want to ask them to measure you. Most health clubs have the machine.
Let me know if you find the correct answer to this question.
Congratulations on reaching your goal!