This is probably a just-so story. Australia is, by the numbers, a high-trust country, and is more diverse than Canada. Both are higher-trust than Japan. Contrary to your claim here, Japan and the US have in fact very similar surveyed levels of trust. What seems actually to be the case is that remarkably high-trust countries, surveyed at, I don't know, several SD above the mean, are rare. Several of them happen to be Nordic, and people extrapolate from there.

I'm sure being ethnically homogenous does smooth things over for Norway. But so does being small and galactically wealthy from extractive industry.