Because most anthropometric formulas such as the Durnin-Womersley skinfold method,  the Jackson-Pollock skinfold method, and the US Navy circumference method, actually estimate body density, not body fat percentage, the body fat percentage is obtained by applying a second formula, such as the Siri or Brozek described in the above section on density. Consequently, the body fat percentage calculated from skin folds or other anthropometric methods carries the cumulative error from the application of two separate statistical models.
I'm a 41 year old bodybuilder currently bulking up, which involves adding a large amount of muscle mass and perhaps some fat too because of the large calorie intake. I read that this is a bad idea because gaining fat will increase the number of fat cells, which will make it hard to lose fat later. So it was interesting to read that the number of fat cells is fixed after about 16 years of age.
I know that the BMI measurement is useless for bodybuilders, due to the much larger than average muscles, so I think monitoring body fat is a much better way to keep healthy.