The north is different due to the weather. I mean, sure, Canadian tundra and the Nordic Arctic and Sub-Arctic is technically land, but I'm not sure you're getting a good comparison of population density by including it in calculations. That would be like calculating the oceans around the US in population density numbers since people live on boats technically.

Nordic countries have small populations due to the limitation of arable land to support humans at all. The US has no such limitation and could support vastly more population.