Trying to map voxels onto a mesh surface will always be tricky. In particular, when you want to map voxels form a scan normalized to MNI space to a very inflated mesh: in this case there is not a one-to-one correspondence. I think the blurring helps reflect the ambiguity of the transform. For accurate mapping you will want to conduct the statistics directly on the vertices, you can use tools like FreeSurfer or PALM that will help you with. In this case, each individual's brain is warped to an inflated mesh and the statistics are run in this space. Surfice is a visualization tool, not a statistics tool, so these operations are handled by different tools.