BF scales don't measure fat directly. What they do is pass a tiny electric current through your body, and measure the resistance. On the principle that 'muscle is wet, and fat is dry', they then estimate your BF%.
Unfortunately, this makes them pretty sensitive to your hydration levels. One salty meal can make a big difference from one day to another in their reported numbers, even in you haven't gained or lost a single ounce of fat.
If you are taking measurements, then you may want to try an online BF calculator such as lowcarbdiets.about.com/library/blbodyfatca
Online calculators also have accuracy problems, but they are considerably less volatile in their estimates than scales.