If I take two women of the same height and one is naturally meant to be BMI 24 and the other is naturally BMI 21, then this is what happens when they both restrict calories:
-The woman at BMI 24 diets down to BMI 20. Technically she is still in the healthy weight range and she receives compliments on her dieting efforts.
-The woman at BMI 21 diets down to BMI 17. People are very worried.
Which one has an active restrictive eating disorder? They both do.
Both women have the same physiological damage and both need intervention to recover and yet only one will likely be identified as having any issue.