Yet this observation of "tightened" predicted scores cannot be fully understood without taking into account the risk of error in the prediction process, as it was visible on the scatter plots in figure 1.
it is also useful to look at the sorted scores, but with their corresponding predicted score (for the same sentence) plotted on the same x coordinate; this what is shown on figure 4, for the wmt13a dataset only (because the phenomenon is the most accentuated in this dataset, and scores conveniently belong to [0, 1]).