Not all citizens at birth are natural born citizens.
See state.gov link at 266 regarding...
Acquisition of Citizenship
U.S. citizenship may be acquired either at birth or through naturalization subsequent to birth.
State.gov...think about it.
A natural born citizen of the United States cannot be anything else. Period. I was born in the United States, my parents were citizens of the United States. No matter how much I might think it cool to have dual citizenship or be a natural born citizen of somewhere else, I cannot, physically impossible because I was born here.
Anyone who can be a natural born citizen of another country or have dual citizenship with another country due to birth cannot be a natural born citizen. Period.