I have to admitt there are some Polish men with flat back of head , but it is not a visible trait among Polish men . It is caused by keeping infants in one position ( on flat back ) and not changing positions too often . But it is more common among older people from rural parts of Poland where the education on such subject was minor .
There must be something old tradition/belief with this, flat-back head, still alive in rural regions. Same here in rural regions in central Turkey. I remember old days in the village when heads of small babies were wrapped by some clothes etc to make backs of their heads flat. I thought it was an old Turkic tradition. Now, I see it is also done in Poland.
By the way, the only Pole I met had a nose with a small curve. I said her she had a curved nose, and she got angry at me. (I thought I was complimenting.)