Hello everyone

I have a fundamental question about RDF profile. I have plotted a RDF profile around a reference point by counting Oxygens in spherical shells in a box packed with H2O molecules.

As the diagram shows after the first peak the fluctuation of diagram disappears around 1.  But when I plotted the RDF by counting the atoms in slab shells from one face of the box to the opposite one a permanent fluctuation is seen in the RDF profile. My question is that why the RDF profile doesn't flatten out around 1 when the reference is a non spherical thing ?

Regards