Facebook designer Caitlyn Winner says she noticed the male icons were symmetrical (except for the hair) but the woman "had a little chip in her shoulder".
The "chip" was the point where the male would go when placed in front of the woman in group icons. This upset Caitlyn. "The woman was quite literally in the shadow of the man."
She also played around with using two, rather than three figures for the friends icon. Finally she just decided to stick the woman at the front.