It has a plausible explanation. People searching for Oppenheimer or Gell-Mann don't type "American scientists", they just type "Oppenheimer" and "Gell-Mann". And a website talking about Oppenheimer doesn't spend much time about his being American---after all, that might be the least interesting fact about Oppenheimer's life.

OTOH, I'll bet there are websites that talk about, say, "George Washington Carver, an American scientist you have never heard about."

That said, the quality of curation seems rather poor.