Furthermore, there is no visible impact of the quality of the MT output, although one would expect the correlation to be lower for low quality sentences: by definition, there are more differences between the MT output and the post-edited version for these sentences, so it should be easier for the system to detect the different level of quality between the two.
it is quite understandable that the system does not detect the difference for an MT output of relatively good quality, but the fact the post-edited version of the really bad translations are also rated as really bad is a major issue.