Box/Wall - Sphere contact with periodic boundaries

Hi,

I'm lost about the contact between box or wall with spheres with periodic boundaries (this is not related to the allowBiggerThanPeriod flag, that I'm using).

Consider 9 spheres lying on a box, in a periodic cell. The cell includes exactly the 9 spheres, and the box may change in size. Its surface may approach the one of the cell, or may be bigger (see script below)

   - if the surface of the box is 90% the one of the cell (set howBig = 0.45 in the script below), I get my 9 sphere-box interactions, fine !
   - if the box fills exactly (in surface) the cell (howBig = 0.5) => only 6 spheres are detected as in contact with the box.. (or 4 if I put radius = 0.5, so that there is normally exact geometrical contact)
   - if howBig = 0.6 (the box extends beyond the limits of the cell) => 6 spheres in contact, for both radii tested (0.5 - exact contact - or 0.55 - overlap)
   - if howBig = 1000 => there are 3 spheres in contact with the box.....

If I use a wall I got 3 spheres-wall contact for radius = 0.5, and 6 for radius = 0.55, but not 9.... May someone explain to me these results ? (or confirm that there is a bug ?)

Jérôme

PS : the script :
# encoding: utf-8

O.periodic=1
O.cell.setBox(3,3,3)
for i in range(0,3):
    for j in range (0,3):
        O.bodies.append(sphere(center=Vector3(i+0.5,j+0.5,1.5),radius=0.55,dynamic=0))

howBig=0.45
O.bodies.append(box(center=Vector3(1.5,1.5,0.9),extents=Vector3(howBig*3,howBig*3,0.1)))

O.engines=[
 ForceResetter()
 ,InsertionSortCollider([Bo1_Sphere_Aabb(),Bo1_Box_Aabb()],allowBiggerThanPeriod=1)
 ,InteractionLoop(
  [Ig2_Sphere_Sphere_ScGeom(),Ig2_Box_Sphere_ScGeom()],
  [Ip2_FrictMat_FrictMat_FrictPhys()],
  [Law2_ScGeom_FrictPhys_CundallStrack()])
        ,NewtonIntegrator(damping=0.2)
 ]

O.dt=1.e-5
O.step()

print '\nhowBig =',howBig,':', O.interactions.withBody(9).__len__(),'interactions with the box\n'