There's another path to tomboyism; Big titties.

Girls who develop breasts early and notably larger are subjected to jealousy-fueled ostracization from female friend groups. The earlier in puberty this happens, the more pronounced the effect is.

Cue male friend groups out of necessity, and thus, the filtering in of male intersests, mannerisms, and generally just a vastly better understanding of men than most women have.

Today, these girls are also fed the same lie that they are in fact men.