I extracted the cortical thickness values using aparcstats2table command and I got the cortical thickness values for 34 regions and mean cortical thickness for each hemisphere. But when I averaged the cortical thickness from those 34 regions regions it was different than the mean cortical thickness obtained by aparcstats2table. I would expect it to be the same or very close but I checked it for more than 100 subjects and they are significantly different. Am I missing something silly here ?

Thank you. 

Regards, 

Nabin