Odd normals pt2

import vtk
raw_points = [
    (0.0, 0.0, 0.0),  # 0
    (0.0, 0.0, 2.0),  # 1
    (0.0, 2.0, 0.0),  # 2
    (0.0, 2.0, 2.0),  # 3

    (2.0, 0.0, 0.0),  # 4
    (2.0, 0.0, 2.0),  # 5
    (2.0, 2.0, 0.0),  # 6
    (2.0, 2.0, 2.0),  # 7
]

raw_polyhedron = [
    [0, 1, 3, 2],
    [4, 5, 7, 6],
    [0, 1, 5, 4],
    [2, 3, 7, 6],
    [0, 2, 6, 4],
    [1, 3, 7, 5],
]

# Add points
points = vtk.vtkPoints()
points.SetNumberOfPoints(len(raw_points))
for index, point in enumerate(raw_points):
    points.SetPoint(index, point)


# Add Polygon
polygon_faces = vtk.vtkCellArray()
for face in raw_polyhedron:
    q = vtk.vtkPolygon()
    q.GetPointIds().SetNumberOfIds(len(face))
    for index, edge in enumerate(face):
        q.GetPointIds().SetId(index, edge)
    polygon_faces.InsertNextCell(q)

face_stream = vtk.vtkIdList()
face_stream.InsertNextId(polygon_faces.GetNumberOfCells())
vertex_list = vtk.vtkIdList()

polygon_faces.InitTraversal()
while polygon_faces.GetNextCell(vertex_list) == 1:
    face_stream.InsertNextId(vertex_list.GetNumberOfIds())
    for j in range(vertex_list.GetNumberOfIds()):
        face_stream.InsertNextId(vertex_list.GetId(j))

# Prepare unstructured grid
unstructured_grid = vtk.vtkUnstructuredGrid()
unstructured_grid.SetPoints(points)
unstructured_grid.InsertNextCell(vtk.VTK_POLYHEDRON, face_stream)

# Get cube normals
cell = unstructured_grid.GetCell(0)
normals = vtk.vtkPolyDataNormals()
normals.SetInputData(cell.GetPolyData())
normals.ComputePointNormalsOff()
normals.ComputeCellNormalsOn()
normals.Update()
print
for i in xrange(cell.GetNumberOfFaces()):
    print i, "=>", normals.GetOutput().GetCellData().GetNormals().GetTuple(i)
# 0 => (-1.0, 0.0, 0.0)
# 1 => (1.0, 0.0, 0.0)
# 2 => (0.0, -1.0, 0.0)
# 3 => (0.0, 1.0, 0.0)
# 4 => (0.0, 0.0, -1.0)
# 5 => (0.5773502588272095, -0.5773502588272095, 0.5773502588272095) -> Odd Result