While crouching, any tools on the players back & any wearable backpacks will shift the same direction, but wont drop with the players back leaving about 2-3 pixels of space.

While standing everything renders beautifully on the back.

However as soon as the player crouches items just sort of hover.
{Unequipped the backpack in this picture so its easier to see}

Heres a screenshot with the backpack

And its hard to tell from this angle but it does do the same thing while riding horses except Im pretty sure it doesnt slant like it does while crouching.