The reason is simple. The smaller diameter clutch inside the bell. The "arms" have to be smaller to fit inside the clutch bell. This means when they engage they can not possibly grab across the whole surface as the diameter is not the same. Only the middle 1/3-ish will make contact with the bell. It would be pretty hard to make a balanced 3 shoe clutch like this with arms that made a full contact.
Again nothing wrong with re-lining the whole surface of the shoe, it is just not really needed. It could help when you wear your pad down that they is no more surface in the middle, the edges may be able to grab at that point. The factory realized this thru their testing and opted to save money. Cuz 10mm less material per shoe x 3 shoes x 30000 bikes can actually end up saving them a bit of cash.