I think the theory is that the middle lane is only used for overtaking slower-moving cars. Perhaps the Swedes, like the Germans, practice good lane discipline and stick to the right until they need to overtake.

I think this 2+1 wouldn't work in the US because (in my experience) very few people use the lanes correctly and they just distribute themselves evenly across all lanes, regardless of speed. That strategy does, indeed, lead to bottlenecks when the number of lanes reduces.