It's a feature
The CubicSurfaceExtractor (without normals) will give the corerct behaviour, but the version with normals is really just to get you started with PolyVox. In the future we will probably get rid of the 'WithNormals' version and add a seperate step to add them to an existing mesh.
And no, in general you should not define an array of volumes unless you really know what you are doing. The SimpleVolume and LargeVolume already stores thier contents as a series of blocks so you already get the benefits of cache effiency, paging, etc. But if you try to implement your own system based on an array of volumes then you will find that voxels do not have access to their neighbours (which may be in a different volume) and this will cause problems for the surface extraction.
