Why are certain parts of my body darker then other parts?

OK well heres the thing.. some parts of my body are darker then other parts. like my stomch and calfs and face are darker then my arms ,hands and legs. I tought it was that the darker parts got more sun but i havnt tanned in a very long time and there still darker. my stomch never gets any sun and it is still the darkest. my hands get the most sun and there the lightest? whats going on here?? haha anyways thanks for helping i just find this really unattractive and wired.