Smaller people have higher resting bpm, and generally women have higher resting bpm than men.
I’m unsure what averages were used in the calculation for sleep phases and awake detection but for women and smaller people it might be placing us in higher sleep phases or mistakenly detecting us as awake due to a naturally higher resting bpm.
There could be a toggle for male/female that allowed the calculation to consider this difference between the sexes and allow women to gather more accurate data.