Fix walls - thickness of the wall

Hello Everyone,

I have a question about the fix wall/gran command - I am using the ver. 3.3.1. of LIGGGHTS.

I am using boundary p p f and I want three walls, two bounding the simulation and one in the middle (to create two layers)

fix zwallsdown all wall/gran model hertz tangential history primitive type 1 zplane ${bottom_plane}
fix zwallsmid all wall/gran model hertz tangential history primitive type 1 zplane ${mid_plane}
fix zwallsup all wall/gran model hertz tangential history primitive type 1 zplane ${upper_plane}

Then, as you can see in the picture attached, in paraview the midwall seems to have a thickness.
How does LIGGGHTS manage this thickness? I can not find information in the fix wall/gran command

http://www.cfdem.com/media/DEM/docu/fix_wall_gran.html

Can anyone reference me to which (eventual) formula LIGGGHTS is using?

Greetings,
Luca