Why do alot of black women have butts that stick straight out? and most white women don't?

Example this black woman http://farm4.static.flickr.com/3134/2678477580_65bd9bb22c_o.jpg

Example this white woman http://www.flickr.com/photos/28701641@N07/2677661259/in/photostream/

is their a scientific explanation on this?
Update: am look at the answers on here, you guys did not help me. you mean to tell me thier is no scientific explanation on this?
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