Why Do We Have Finer Hair in the Hairline?

Why in the very front of the hairline the hairs are not the same thickness, why are some of the me finer than others?

Every person should have finer hair in the frontal hairline. I have measured the thickness of the hairs at the frontal hairline, those hairs about an inch behind the hairline, and the hair in the back of the head (donor fringe area). The frontal hairline hair is thinner. A patient of mine called it “angel hair” and it is this hair that allows the gradient from the bald forehead to the thicker hair behind the hairline. That is the way it is, genetic of course.