Note that by default SPM normalizes to an average sized brain, while most other tools (ANTS, FSL, etc) average to the MNI template which is larger than the average sized brain, you can see this nicely in Figure 1 of Horn et al. (2018). This is why MRIcroGL includes both the mni152 and spm152 template. Furthermore, I think it is always challenging to project statistical maps from voxels to mesh-based meshes. If you calculate values in statistics, I think you should consider voxel-based volume rendering, like MRIcroGL. If you compute your statistics based on per-vertex data, use a mesh based visualization tool like Surfice.