American exceptionalism probably affects this quite a bit, I would imagine, in terms of biasing US residents to overestimate their own happiness because of a skewed understanding of what the rest of the world is like by contrast
It's actually pretty common and one of the hurdles of getting comparable data.
An anectode from an Amartya Sen book: Uneducated women in rural India, when asked anout their health status, effectively said everything is fine. After getting access to better healthcare and health education, self reported health scores went down. Classic case of unknown unknowns: The women didn't know that live actually could be better than what they experienced so far.
The same is true for this discussion: I'd wager that thanks to Hollywood and world news coverage, the average citizen of a small EU country like Norway has (or at least thinks they have) deeper knowledge of live in the US than the average US person about live in Norway. So the Norwegian, when asked about their own happiness, can position themselves in context of this knowledge.
