Kgw, there are a couple of disorders where osteoporosis can occur first in the radius one is hyperparathyroidism and the other is celiac disease.
If you have slender arms, the bone wouldn't be less dense, but would appear less dense on Dexa because Dexa cant determine the size of the bone it measures.
There could also be a mixup in which part of the arm is scanned for the T score comparison. Between ultra distal, distal and mid forarm there is a difference in bone type which determines density.