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My thought is that for some reason, with the Canadian and US proximity, the algo could see the US page as stronger. Do you see in Analytics for the few times this happens, the organic traffic to that page, is it border Canadian towns? It's definitely not perfect with bordering countries, but if you are geo-targeted correctly and the content is different, then overall you should not have major issues. If you'd like, tell me a few terms and your domain and I'll check it out.