Please tell me why my hair is darker at the top than it is at the bottom? I'm only 23 and no, it's not gray!?

I'm a 23 year old African American female and I have noticed that my hair is 3 different colors: jet black, dark brown and medium brown. My hair is all natural meaning that I don't have any relaxers or any other chemicals in it. I wash it every 2 weeks with biotin enriched shampoo and I use hair mayonnaise as a conditioner. I also use Ginseng Oil to moisturize it 3 times per week. I take biotin, Vitamin E, Vitamin C, cod liver oil, folic acid and a hair, skin and nail vitamin every single day. I try not to stress and I get at least 8-9 hours of sleep per night. So what the hell is wrong with my hair? And no, it's not gray, not even one strand of it.