Body fat scales use a technique called Bioelectrical Impedance Analysis (BIA) where a small and completely harmless electrical current is passed through your body (you feel absolutely nothing at all). The electrical current passes more quickly through fat free tissue like muscle than it does through fat or bone tissue. As we all know, water is a very good conductor of electricity, and since muscle is 70% water and fat contains little water, the body fat scale is able to make a distinction. This means that the amount of resistance to the electrical current from your body relates to how much fat-free mass you have and therefore to your body density.
Like all body fat tests, body fat scales don't actually measure your body fat percentage. They determine your body density. The scales (or the Doctor etc in other methods) then use a formula to calculate body fat percentage based on body density.
The consistency of the measurements from any formula is a bit questionable. Changes in water retention (which we all know can be a real s*d on this diet!!), food intake, skin temperatures and minor posture changes among other things can affect the readings given by these devices. Any variable when readings are taken can give you with some perplexing numbers - just as you are finding S4L.
However, even though these scales are not as reliable as skinfold measurements methods (at the gym for example), they are better than not getting a number at all.
For best results, try to keep all factors as close to identical each time you jump on your body fat scales (temperature, posture, time of day, water intake etc.). For example, when I used to use them, I did not drink water for about two hours before ..... to minimise the unnecessary water onboard.
