Problem with super-lattice

Hello,

I define a super lattice composed of two distinct holes (radius R1 and 
R2), my problem is that diagram band is not equal for R1=R2 and R1 ~ R2.

For example :

(define-param R1 0.15)
(define-param R2 0.15)

(set! geometry-lattice (make lattice (size xsize ysize no-size)
                         (basis1 1 0)
                         (basis2 0 1)))

(set! geometry
    (list
    (make cylinder (center -0.25 -0.25 0) (radius R1) (height infinity) 
(material air))
    (make cylinder (center +0.25 -0.25 0) (radius R1) (height infinity) 
(material air))
    (make cylinder (center -0.25 +0.25 0) (radius R2) (height infinity) 
(material air))
    (make cylinder (center +0.25 +0.25 0) (radius R2) (height infinity) 
(material air))
))

==> I get this result :
<http://juliendusud.free.fr/band2.png>

But if I replace (define-param R2 0.15) by (define-param R2 0.1499) I 
get <http://juliendusud.free.fr/band1.png>