I wonder how frame size adjustment works with this. As a small-framed Asian woman, I am already at the "close-to-underweight" BMI level with plenty of fat on my stomach and nontrivial quads... I find BMI calculator accusations of underweightness pretty offensive, because I know muscle weighs more than fat, and I am proud of my muscle weight.
Some ideal weight calculators advise adjusting the ideal weight range 10% up or down according to frame size, which puts me back in the solidly "normal" category, so that's what I pay attention to. Sometimes, we weigh more or less just because we have more or less bone underneath.