I've noticed that many people I've talked to use the term "American" to mean "white Caucasian." For example, I was watching a cartoon with an acquaintance from East Asia, and he remarked that a light-skinned, blond-haired, blue-eyed character looks 'American.' I was like, "Er, there are many people in the world who look like that and they're not American," whereupon he qualified his statement as "I meant 'European'."
The idea that 'a person of European appearance'='American' not just ignores the existence of many Americans who are not people of European descent and who not look European, but also ignores the existence of people of European descent who are not American - many Europeans and members of the European diaspora in the Americas would fit that physical description. And some West Asians and Central Asians have those physical attributes too. So, a blond, blue-eyed, light-skinned individual could have likely been Armenian, Russian, German, Canadian, Brazilian, and a long list of nationalities. I suspect this idea that "white Caucasian"="American" came about because of the dominance of Hollywood movies overseas. Sure, people in East Asia have seen movies from other places like Russia, France, Germany, but these films don't have the volume and reach of American movies.
Many of us in America understand 'American' as a label that denotes nationality, not race. But even some Americans of color have subconsciously internalized the idea that 'American' and 'white Caucasian' are equivalent. I've heard an American of African descent refer to somewhat European-looking Japanese animation characters as an "American style" characters, the implication being "white Caucasian = American" (which is not quite the same as "American = white Caucasian" but just as erroneous). Could it be that some of us have in some way accepted and perpetuated our own exclusion?