I knew of this, but it's the first time I've read it where it's explained for non-medical
"The issue is that BMI measures health risk by calculating height and weight. However, muscle and bone weigh more than fat, so BMI measurements can overestimate the danger for people with a muscular build or a larger frame. ....Bioelectrical impedance analysis, or BIA, uses undetectable electric currents to measure not only the percentage of body fat but also lean muscle mass and water weight."
I believe The Villages Health has this machine and that I used it some time back
BMI is B-A-D, a new study suggests. Here’s a better way to measure weight