Yeah, a bit of diffuse thinning in the middle there.
Was it there since childhood?
Did you have that area traumtized in any way?
You may just have the hair lenght that amplifies that. It is possible that if you cut shorter it would be way less visible, or if you grow the hair longer, it most probably would be hidden.
Do your head bones have any irreglarity in that area, some sort of bump or "valley"?? Yes, it may sound strange, but i have a theory...