These two pictures illustrate why I think the rear seats may go further back in 50:50 cars compared to the five seater. Of course, one is a graphic and the other a photo, so not exactly scientific, so until someone measures up, we won't know for sure...

(Ignore this bit as only theory for now: so, if it turns out that the 4-seater has a shallower parcel shelf, this may in turn explain why they don't fit the strings, as I guess using the same strings could pull the shelf out of its hinge at the end of travel - a problem that you would not get with a deeper shelf. Of course this could be sorted by different attachment points etc etc...)