@daisy17, I see the sense of what you are saying.
But, the dexa score would be the same no matter the size of the bone. The dexa score might be sufficient in a small bone where the same score would be insufficient in a larger bone. So, without calculating for bone size, a person with small bones might be (over) diagnosed as osteoporotic.
A large glass of water needs more sugar to be as sweet as a small glass of water. If you count the particles of sugar in a glass without assessing how full the glass is the count of sugar particles doesn't account for the dilution.
Ten particles of sugar in a full glass will be less sweet than ten particles in glass that is half full.
TBS uses a grey scale to calculate the space between particles giving us a fracture risk based on homogeneity.