Dear Freesurfer Friends

I have a question about cortical thickness. I do not understand why these commands give us DIFFERENT values for "average cortical thickness". Can you please let me know if you have any idea. Thanks a lot

mris_anatomical_stats N2207 lh number of vertices = 157417 total surface area = 102378 mm^2 total gray matter volume = 234251 mm^3 average cortical thickness = 2.080 mm +- 0.889 mm average integrated rectified mean curvature = 0.198 average integrated rectified Gaussian curvature = 0.223 folding index = 13893 intrinsic curvature index = 1137.3

mris_anatomical_stats -t N2207/label/lh.cortex.label -b N2207 lh same as mris_anatomical_stats -t -b N2207 lh number of vertices = 157417 total surface area = 102378 mm^2 total gray matter volume = 0 mm^3 average cortical thickness = 0.000 mm +- 0.000 mm average integrated rectified mean curvature = 0.198 average integrated rectified Gaussian curvature = 0.223 folding index = 13893 intrinsic curvature index = 1137.3