It's straightforward to measure a person's weight. It's much harder to measure how much of that body weight is fat rather than lean body mass (tissues such as muscle). People come in varying shapes and sizes, with differing proportions of muscle, bone, fat, and other tissues. Body fat estimates are based on taking a measurement and then applying a formula that makes assumptions about body composition.
Also featured on the HHMI DVD Science of Fat, available free from HHMI.