It seems to me to be a form of delayed pigmentation. Being born with little or no color and gradually get more color as they grow.
Chicks both sexes start out with very little color- yellow down with or without cream tint to wing feathers at hatch.
Both sexes get color at roughly the same rate as they grow through their first weeks.
Females(all Indias, no matter if IB, cameo, opal etc) get their adult color/pattern very early, by the time they are 6 months old they have fully the adult hen coloration/pattern. My guess is the progress of coloring/pigmentation is "stopped" in females this early. So they are stuck in the "middle" of the black shoulder gradual pigmentation process for rest of their life.
Males in all colors continue to develop changes in color and patterning- more pigmentation through their young years until 3,4 yrs. So they(black shoulders) continue to get "fully colored" unlike the hens.
My guess, if the case were peacocks matured in their plumage much sooner, say, one year then perhaps they might have resembled the black shoulder hens a little more in how much white they have. As in they would have looked like a yearling black shoulder male(with longer tail feathers) for the rest of their life.
If it were the case peahens went through the same color progress/changes for a much longer period of time(say 2 or 3 yrs), then they possibly would have ended up solid black. Because in this case the pigmentation progress got to continue instead of being "arrested" so young.