I think it's an attempt to express that the station consists of only two segments: Russian (ROS) and US (USOS), but the US invited its allies to work together on its segment. So parts of the USOS are made in Europe, Canada and Japan, and generally lifted to space by the US, usually on the Space Shuttle.

(All this was pretty lucid of the US, but obviously the Russians did no such thing on their side. The Japanese even managed to get an ISS resupply mission launched on their own vehicle, which is no small achievement, and the ESA did a bunch of good science. And what would space be without the Canadarm :-)