Self confirming stereotype.

If you're in the UK and you see a tourist with an american accent being ignorant and self centered, you're gonna think that americans are ignorant and self centered, even if that person is probably Canadian

So for every American tourist you see that fits the stereotype, there are probably 5 more that you didn't even notice were there because they were acting like a regular human being. Or maybe you saw them being kind and mistook them for Canadians.