I choose the same height, age and weight of these three races which are whites, blacks and asians. All of them are in their 20s. I found out that Black people has the longest and widest penis ( not erect) followed by whites and the smallest would be the Asian. My question why do penis sizes differs from ethnicity? Aren't we humans are the same? What is the cause of it?

I am doing a research on anthropology. Uni student