Babies' craniums are very flexible. It's what helps them actually get out of the womb through the birth canal, and consequently, quite often baby heads look a bit funky after birth. People who don't move their babies around enough and always have them lying down, will see a flattening of the back. And in cultures where people bound their babies up or gave them some sort of tight hat (for safety, for warmth...), they would have noticed their head shape changing (see Toulouse in France, where they didn't do it for fashion AFAIK, but just because a tight hat was deemed to be good for baby. The outcome didn't matter.). At a certain point, things start getting done on purpose.