Home, low cost, BIA (Bioelectrical Impedence Analysis) devices are not as accurate as professional devices for a number of reasons. Some only measure across the feet, whereas others across the hands only. Those that measure from hand to foot are more accurate.
When measuring from hand to hand, the signal travels across the chest, so lower body composition is excluded. Likewise, in foot to foot BIA devices, the signal only travels through the legs and ignores the upper-part of the body composition. Depending on your body shape, the results can be very misleading.
Body composition results greatly depend on your weight. This can change throughout the day depending on hydration and bowel content. Recent activity level also affects the results. This is because blood in your body tends to pool in your legs when at rest or standing up. Muscle contraction during exercise redistributes the blood throughout your body, by squeezing venous blood vessels and thereby changes the impedance values used to calculate body composition.
All of these factors can impact on the results a BIA device outputs. FitQuest's BIA averages your test scores so by using the same device consistently, you are able to track your progress more accurately.