5.4 leading up to Table 5.4.7 on p. 167) that the Wilcoxon signed-rank statistic W provides greater robustness and often much greater efficiency than the paired T, with ARE which is 0.95 with f a standard normal density, and which is never less than 0.864 for any symmmetric density f .
in our context, continuous scores such as pyramid exhibit document-specific score differences between summarizers which often have approximately normal-looking histograms, and although the alternatives perhaps cannot be viewed as pure location shifts, it is unsurprising in view of the ARE theory cited above that the W and T paired tests have very similar performance.