Why is the ratio of H2/acetate over 2 in glucose fermentation by Caldicellulosiruptor saccharolyticus?

Why is the ratio of H2/acetate over 2 in glucose fermentation by Caldicellulosiruptor saccharolyticus?

Database: Elsevier - ScienceDirect (ساینس دایرکت)
Journal: International Journal of Hydrogen Energy - Volume 38, Issue 26, 30 August 2013, Pages 11241-11247

Nan Shen, Fang Zhang, Xiang-Ning Song, Yan-Shan Wang, Raymond J. Zeng,