error when re-loading mesh from file

The error is this.  If I create a 3-D mesh from scratch, with some uniform refinements, it has a certain number of faces, facets, and edges.  I then save the mesh to a .vol file.  When I reload the mesh, the number of faces, facets, and edges is *different*.  Why?

I have created a sample code below.

---

import numpy as np

import netgen.gui
from ngsolve import *

from netgen.csg import unit_cube
from netgen.csg import CSGeometry
from netgen.csg import Pnt
from netgen.csg import OrthoBrick

#load_mesh = True
load_mesh = False

mesh_FN = "test_mesh.vol"

if load_mesh:
    print("Load mesh from file.")
    mesh = Mesh(mesh_FN)
else:
    print("Create mesh from scratch.")
    # make the mesh
    cube = OrthoBrick( Pnt(0,0,0), Pnt(1,1,1) ).bc("Gamma")
    geo = CSGeometry()
    geo.Add(cube)
    mesh = Mesh(geo.GenerateMesh(maxh=1.0))
    for r in range(3):
        mesh.Refine()
    # save the mesh
    mesh.ngmesh.Save(mesh_FN)


print("Some info about the mesh:")
print("Number of Elements: " + str(mesh.ne))
print("Number of Faces: " + str(mesh.nface))
print("Number of Facets: " + str(mesh.nfacet))
print("Number of Edges: " + str(mesh.nedge))
print("Number of Vertices: " + str(mesh.nv))

---

When you run it the first time, you should get:

>From scratch:
>Number of Elements: 3072
>Number of Faces: 7530
>Number of Facets: 7530
>Number of Edges: 4905
>Number of Vertices: 729

Next, change "load_mesh" to True and rerun.  The output will be:

>When loading from file:
>Number of Elements: 3072
>Number of Faces: 6528
>Number of Facets: 6528
>Number of Edges: 4184
>Number of Vertices: 729

Why is it different?