I have a body fat monitoring scale and have had wildly varying results over the course of a couple of days.
I believe this is due to varying hydration levels. It was only like 50 bucks, so I guess you get what you pay for.
Also, you need a very good electrical "connection" to the scale. Try wetting the bottoms of your feet, or mixing a salt solution that you can spray on. You should get much more consistent readings