There can be a lot of reasons. First, just because something is in the same country doesn't mean it's close. Conceivably, people in western Canada might import a resource from Washington State, and people in Maine might import the same resource from eastern Canada. So it looks like Canada and the US are just swapping resources pointlessly, but it's actually easier and more convenient for everyone to do it that way. Another possibility is that the same resources might be available at different times of year. Maybe the strawberry crop comes in from May to July in Mexico, and from July to September in the US. So in the earlier part of the year, Americans import Mexican strawberries, and later in the year, Mexicans might import American strawberries. Another possibility is that there are differences in the resources available in the respective countries. Americans import Swiss chocolate, which is a fancy luxury good. Swiss people may import American chocolate, which is a cheaper, tasty bad e market candy. If you just see a spreadsheet that is labeled "chocolate", it sounds pointless, but it's really fairly different stuff.