The problem with body fat scales is that many factors exist (e.g., how hydrated you are, when you last ate and exercised, the quality of the scale itself, etc.) which can cause inaccurate measurements in body fat. For instance, the scales may underestimate those with higher body fat and overestimate those who are leaner. Also, the body fat scales tend to be inaccurate in elderly people, athletes, children, and people with osteoporosis.
BOTTOM LINE: If you do use a body fat scale, it's recommended that you utilize the body fat scale as a means to track body fat changes over time (i.e., on a monthly basis) rather than as an absolute value. To obtain more accurate relative changes in your body fat, it's recommended you step on the scale upon wake-up with no clothes on before eating any food. Remember: focus on changes over time rather than any single measurement.
Brian Danley, CFT
"Motivation is what gets you started. Habit is what keeps you going."