Oh, I see where you got confused. When people say 'Americans do something' they mean 'most Americans do it' or 'significant portion of American population do it', not 'all Americans do it'. You see, countries are big and there are a lot of people in them and it very rare for everyone in a country to think exactly the same. That's why we usually focus on the prevailing attitudes that shape the country over the years instead of looking for believes shared by ALL citizens (because they don't exists). Currently over 40% of Americans believe that "God has granted America a special role in human history" and it was way higher in the not so distant past, that's why someone could say that "Americans believe it". Hope this helps.