From my VERY limited knowledge gained from reading tidbits and exploring myself around Skyrim modding, it must happen during the item's placement on the character skeleton.(?) Perhaps it's intentional. Many of the shields curve inwards along the edges. If they place the Shield so the center straps touch the Character arm, then the Shield may clip into the arm as well, making the visual worse.

When it comes to fixing this, it's two fold. All visual art released to the Live Build are Final, and will only be changed if it breaks normal gameplay. If it is going to be changed, it's WAY down their priority list.