I have the explanation, though I dont think it actually makes any sense.
For whatever reasons standard fat % guidelines are as follows (for women):
Body Fat Rating Scale
Exceptionally Lean 10-15% **
Very Lean 16-20% **
Lean 21-25%
Moderate 26-29%
Overfat* 30%+
*Women are borderline at 30% and clinically obese at 35% body fat.
**A lower body fat percentage for women increases chance of amenorrhea (cessation of menstruation).
These guidelines suggest that for all women the ideal range is 26-29, though I personally have a strong disagreement! Most trainers and drs would prefer to see you in the 20-24 range, but it varies a lot by person.
Im 23% at 140lbs and 5'6 and at 121 I was about 17% which was maybe slightly too low but I never had any 'underfat' symptoms such as period cessation. I dont have boobs either, so I feel like that allows me to carry a lower body fat % more safely. It is really up to you and how comfortable and healthy you feel at any level below 29%