If the biography in Wikipedia is accurate, he was in the US until the age of 6, and then was in the US from the age of 10. Bottom line is that it should be no mystery.
no mystery??
You must have never met anyone who was raised overseas
from age 6 to 10, in a foreign land, he would have picked up an accent of some kind
I know adults who pick up foreign accents of England and Australia who move over age 20 and spend one single year, you hear it in different words
but for a CHILD from age 6 to 10 to NOT pick it up...THAT is the mystery...