Tanita vs. DexaScan

I didn't get info on water or organ weight.  Though the BMI did print out.  Did she say why a normal BMI would be unhealthy for someone post-surgery?  I realize BMI doesn't take into account muscle mass or frame size, but I don't think being in the normal range would necessarily make someone unhealthy.  I've always understood it to mean there are plenty of people who don't hit that range who are doing just fine.