In general, this quiz makes not too much sense...
That being said, Mexico should definitely be higher up because most "mestizos" come from centuries of interracial marriages. That's possibly true of some of the other Latin American countries here.
Plus, it's worth pointing out that most people from most "racial" groups in America have European ancestors, including Hispanics, African-Americans, and Native Americans. If you take those peoples into account and not just "white people", the US's percentage would probably go up.
If you count someone with 2% European ancestry as being of European descent, you have to count all African Americans as well.
- The most useful clarification ever.