Volume above and below the auto-close pane is based on an auto-closed Mesh. First the points along the largest open area of the mesh are used to create a best fit plane; the mesh is closed that plane, and then volume of the closed shape is computed. If a Mesh shape does not have a planar opening, the auto-close may not seal the whole mesh, in which case, a portion of the mesh may remain open. The measurement will therefore show a volume above and below the auto-closed pane.