I don't believe these scales work. If it's really the electrical current that is measuring your fat percentage, then why do you have to enter your age and sex? I tried changing both of these to see what would happen and this resulted in a totally different fat percentage! Try this with yours and see what happens.
Also, if you eat less carbs than average, the fat percentage that they show is much too high. For example for me (woman) it shows 35 percent (= obese), which is not possible, as my BMI is only 22. This error is probably caused by the fact that when you are eating less carbs, your body retains less water.
Anyway, I think the best way to keep track of changes in bodily composition is simply to measure your waist, hips, etc.
The Master Cleanse or any other way of eating where you reduce your calories will always slow down your metabolism somewhat. However, this is completely reversible if you are careful to ease gradually back into a normal eating pattern and don't start eating way too much to "compensate" for what you've missed...