Much shorter version:
Say, through the mischance of illness, birth injury or genetic disorder, 1% of all children are born with something which damages body and mind, such that they look odd in some way, and fall below IQ 70.
Say, through the normal variation of genetic inheritance some children with normal undamaged bodies and minds also fall below IQ 70.
Then in the case of white kids the proportions will be 1% funny looking and 2% normally backward, so 1 in 3 looking odd.
In the case of black kids the proportions will be 1% funny looking and 16% normally backward, so 1 in 16 looking odd.
Black kids below IQ 70 will usually be normal looking, white kids less frequently so.