E.g have this kind of graph (see Scenegraph.jpg below):

Layer
-Cube
--Cylinder
-Sphere

1) Moving Sphere under cylinder is impossible.
2) Moving Sphere directly under cube messes up the graph, see Move_Sphere_1 & 2 below. Sphere appears just under the cube though it is child of the layer, and the children of the cube appear as if they were children of the Sphere.