Thanks to Marino, I have found the reason for the difference in our bending stiffness calculation. The original discussion is here:
http://imechanica.org/node/4029

The reason why we have a higher bending stiffness is due to the dihedral term. This dihedral term does have a significant contribution to the bending stiffness. However, in Ref. [26], apparently, this dihedral term was ignored.
I have written a short document showing the contribution of the dihedral term to the bending stiffness. Please take a look at the attachment.
I received great help from Dr. Huang and Marino. Thank you very much.

Qiang Lu and Rui Huang

Department of Aerospace Engineering and Engineering mechanics, University of Texas, Austin,
TX 78712, USA