[Freesurfer] Subcortical Volume Calculation

Dear Freesurfer users,

I am wondering how are the volumes calculated from stats such aseg.stat. When I export the aseg and do a voxel count and multiple the volume per voxel, my total volume is different from that is given by the stats. For example, the hippocampal volume given by aseg.stats for one of my controls is 4000 cm3, but when I export the aseg.mgz into matlab and do a voxel count (my original voxel volume is 1mm3 isotropic) for that labeled mask, the total volume is aorund 3000 cm3. Does freesurfer does some sort of intensity normalization or partial volume correction? Even if so, the magnitude of difference is large that I am puzzled.

Thank you very much.