Domestic exports and total imports

I'm trying an experiment dividing the country into two regions, one large and one small. (I'm using the 2013 data and the online tool.) I'm looking at the commodity trade data in the two regions. For many commodities, the domestic exports for the large region exceed the total imports (foreign and domestic) for the small region by quite a bit. (I wanted to offer DC as the small region and the 50 states as the big region as a concrete example. You do see this happen there, but the differences are small. For parochial reasons, my first experiment used the Detroit MSA--Lapeer, Livingston, Macomb, Oakland, St. Clair, and Wayne counties in Michigan as the small region, and the other Michigan counties, the other 49 states, and DC as the large region. In that case, domestic exports of the large region frequently exceed total imports of the small region by multiples of up to 6 times / by up to several billion dollars. In fact, Detroit's total imports are $218 billion, while the rest of the country's total domestic exports are $259 billion.) (I note that domestic exports for a model built from the 50 states and DC are not quite 0, but are quite small. Domestic exports for the standard US model are even smaller.) What's going on here?