Why do black computer cases get white scuffs and why do white computer cases get black scuffs?

Here's a bit of a mindbender question that I've come up with.





Pretty much what the title says: ';Why do black computer cases get white scuffs and why do white computer cases get black scuffs?'; This is what I mean:





Black computer case with white scuffs:


http://img390.imageshack.us/img390/6742/鈥?/a>





White computer case with black scuffs:


http://img257.imageshack.us/img257/1877/鈥?/a>





I've cleaned both of these cases very thoroughly but they won't come cleaner than they are in the pictures.





I'd love to hear some of these answers!Why do black computer cases get white scuffs and why do white computer cases get black scuffs?
Black computer: my guess is that the scuffs are actually scratches in the plastic, which make it appear lighter because of the different texture.





White computer: rubber or black things rub against them, and with time the scuff marks get harder and harder to get off.Why do black computer cases get white scuffs and why do white computer cases get black scuffs?
Uh,, you can't see the ';white scuffs'; on the white towers and you can't see the ';black scuffs'; on the black towers..