I was born in the US in 1974 and have had a dime sized bald spot (white in color) at the crown of my head my entire life. Oddly enough, my daughter, born in 2000 has an identical bald spot in the same exact spot. She is due to give birth to my first granddaughter in 2 weeks and we are both curious to see if she has one as well. No clue why either of us have them.
Based on this history, I suspect it is an unusual inherited bald spot.