Getting different results for equvalent supercells

Hi all,

There is a simple code below for a triangular lattice photonic crystal 
slab of holes.   I have three different ones and the only difference 
between them is the position of the slab in z dimension.  In the first 
the offset of the slab center from z=0 plane is 1, in the second the 
offset is 1.5 and in the third it is 2.2.  So the only change is in the 
definition of the slab center.  (One is (0 0 1), one is (0 0 1.5) and 
one is (0 0 2.2)....)

The band diagrams of the first two are similar but quite different 
compared to the third.  And the one which is more close to the expected 
band diagram is the third one.

So why do I get different diagrams although you will end up with the 
same crystal structure if you replicate each of the three supercells in 
all three dimensions?  What am I missing?

Regards,
Erman Engin

The Code for the third:
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(define-param r 0.25) ; the radius of the holes
(define-param supercell-h 4) ; height of the supercell
; triangular lattice with vertical supercell:
(set! geometry-lattice (make lattice (size 1 1 supercell-h)
                         (basis1 (/ (sqrt 3) 2) 0.5)
                         (basis2 (/ (sqrt 3) 2) -0.5)))
(set! geometry
      (list (make block (material (make dielectric (epsilon loweps)))
    (center 0 0 0)
    (size infinity infinity infinity))
    (make block (material (make dielectric (epsilon epsGaN)))
    (center 0 0 2.2) (size infinity infinity hGaN))
     (make cylinder (material air)
    (center 0) (radius r) (height infinity))))
; 1st Brillouin zone of a triangular lattice:

(define Gamma (vector3 0 0 0))
(define M (vector3 0 0.5 0))
(define K (vector3 (/ -3) (/ 3) 0))
(define-param only-K false) ; run with only-K=true to only do this k-point
(define-param k-interp 2)   ; the number of k points to interpolate

(if only-K
    (set! k-points (list K))
    (set! k-points (interpolate k-interp (list Gamma M K Gamma))))

(set-param! resolution (vector3 32 32 32))

(set-param! num-bands 9)
; Run even and odd bands

(run-zeven)
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