Two people of identical height and weight can differ enormously in how their bodies are built. The scale reports a total, and the composition of that total is what most health measures actually depend on.
The compartments differ in density and function
Body mass is conventionally divided into fat mass and fat-free mass, the latter including muscle, bone, organs and body water.
Muscle tissue is denser than fat, so a given mass occupies less volume, which is why measurements and appearance can change while weight does not.
The compartments also behave differently: muscle contributes to resting energy use and to physical capability, while fat's effect depends heavily on where it is stored.
Location matters more than quantity
Fat stored around the abdominal organs is metabolically active and releases substances into the circulation reaching the liver directly.
Fat stored under the skin, particularly on hips and thighs, carries a weaker association with metabolic disease at the same total mass.
This is why waist measurement adds information that weight alone does not, and why two people at the same weight can have quite different risk profiles.
Methods estimate rather than measure
Bioelectrical impedance devices pass a small current through the body and infer composition from resistance, which depends heavily on hydration.
Skinfold measurement samples subcutaneous fat at set sites and converts it through equations built on particular populations, so results vary with the operator and the equation used.
Imaging methods separate tissues far more directly, which is why they are used as the reference against which the cheaper methods are validated.
Consistency beats accuracy for tracking
Every method carries a systematic bias, meaning it may read consistently high or low for a given individual.
Because the bias is fairly stable, the change over time from the same device under the same conditions is more trustworthy than any single absolute figure.
Comparing a reading from one device against one from another mixes two different biases and produces a difference that reflects the equipment rather than the body.
Why composition explains conflicting results
A person beginning resistance training while reducing intake may gain muscle and lose fat, producing little movement on the scale despite substantial change.
Interpreting that as failure is a common reason people abandon a programme at the point it is working.
Tracking several indicators together — strength, measurements, how clothes fit and a rolling weight average — gives a far better picture than any one of them, and clinical assessment remains the route for anything that needs a definitive answer.