Volume(mm^3) from voxels

Please may i know Why the volume statistics from the segment statistics take volume as
volume = no. of voxels * pixellength * pixelwidth * slicethickness* 2 and not
volume = no. of voxels * pixellength * pixelwidth * slicethickness

Is there anything i am missing while calculating volume from voxels (only slicethickness or something else to be considered )