The reason for this is a scale tells you absolutely nothing of importance. This is especially the case if you include exercise as part of your body fat loss program. The scale can't tell you what the weight loss is made up of. You're wearing smaller clothes, you're looking great in the mirror, and all your friends want to know your secret but the scale says you're a failure. In other words, a pound of muscle is smaller than a pound of fat.
Body fat is expressed in percentages.
A simple method to measure body fat percent is with skin-fold calipers. The measurements are then compared to a chart that then gives the fat percent. Such calipers are readily available from fitness centers. A more sophisticated method is the water tank. Since muscle is denser than fat, then the more muscle a person has the more they will sink.