What does people think about the current mental health guidelines for sufferers of anorexia?

Men are naturally heavier than women. So men have, on average, a higher body mass index (BMI) than women. So why do women and men get sectioned if they have anorexia and their BMI reaches 13 BMI or under. Surely if men are heavier, men are more underweight if their BMI is 13 than women would be at 13. Surely the BMI limit for men should be a little higher than womens BMI limit.
because males are naturally heavier, shouldnt males underweight BMI limit to be hospitalised should be slightly higher than females. Surely if men are naturally heavier, they are more underweight than females if their BMIs are 13