In the paper (Conroy and Dang, 2008) the authors noted that while there is a significant gap in performance between machine systems and human summarizers when measured by average manual metrics, this gap is not present when measured by the averages of the best automatic metric (ROUGE).
in many settings, the null hypothesis comprises many possible probability laws, as here where the null hypothesis is that the underlying probability laws for the score-samples of two separate summarizers are equal, without specifying exactly what that probability distribution is.