[Freesurfer] bbregister surf2vol

Dear Freesurfer I want to use surf2vol to get volume files of gray thickness.And the --volreg Contains the matrix that maps XYZ in the reference anatomical to XYZ in the functional volume. So I use bbregister.Why I need not invert the BBR registration matrix with surf2vol?I mean , the BBR registration matrix is functional to anatomial volume,the surf2vol need anatomical to XYZ And I use bbregister --s sub1 --mov /sub1/mri/orig.nii --init-fsl --reg/sub1/register.dat --t1;mri_surf2vol --surfval /sub1/surf/lh.thickness.fwhm0.fsaverage.mgh --hemi lh --surf pialfs --volreg /sub1/register.dat --outvol ./ --template /sub1/mri/orig.nii the resluts are 256 volumes.Why this happen?Should the voulme be 1 file?