See a multi-part series on body fat testing, and its inaccuracy on an individual level. http://weightology.net/?p=235
Many people don't realize how inaccurate various body fat testing techniques can be on an individual basis, even when tracking the same individual over time. The first article discusses how body fat measurement is a prediction rather than a measurement. The second article discusses hydrostatic weighing and the limitations of a 2-compartment model.
