It is ideal to use Surfice for statistical overlays that are calculated based on mesh vertices rather than voxel intensities. For voxels, you need to project the voxel location onto the mesh, and this will not be accurate, in particular for dilated meshes. Since the voxel to mesh projection is only for illustrative purposes, the implementation is pretty crude. It will either be based on the voxel nearest the vertex (nearest neighbor), or it will compute the average (trilinear interpolation) of the eight voxels nearest the vertex. The choice between these is controlled by "Smooth voxel-based images" in the preferences window. The code is open source, so you can always implement more sophisticated methods. Alternatively, you could convert the voxels to mesh vertices using another tool that saves data in a fomrat Surfice supports, like GIFTI.