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