That makes sense, if the wheels stick out further than the arch's then they won't pass type approval. Cue plastic bits of bodywork, as the arch, or at least part of it needs to be wider than the wheels.
However, I don't think that's the reason here for 2 reasons, one is that the wheels don't stick out further than the arch's, and there's muck trails where the water from being on the motorway has seeped out of the arches off the wheels and then run to the back of the car on mine.
Looks like it's to keep things clean to me!