Answer:
Disagree, but maybe that's because i'm white.
See, you live in America and White people there are a big mix of all european ethnicities, so there's a common look for you to see.
I'm from Europe and have traveled around, so I can spot certain features that can tell apart two white ethnicities.
I'm Scottish, but most people would not think it right away because I don't have red hair, or freckles etc. (it's just a stereotype, but if you saw it you'd be able to identify that as a celtic trait)
I can tell a polish or Latvian person, here in the uk, just by their facial features.
Other times i wouldn't be able to guess that someone wasn't from my country until they start to speak and I hear a different accent.
It's just like in Asia, Asians would find it much easier to tell between a chinese person and a korean person etc.
Or in Africa, Black people would find it easier to tell the difference between a Nigerian and somalian...
In America, it's just a huge mix so people lose some features that would have told you apart otherwise.
If you ever travel around more, you'll soon learn the differences.
Edit: I think the reason that White Americans will say what a mix of Ethnicities they are is because as humans, you want to know who you are, where you came from etc.
Obviously not being native to America, you can't claim that as your ethnicity so they just want something to relate to.
However, I believe everyone should relate more to their Nationality so people should just be happy saying 'American'
