Dangeling tail ends.
It seems to me that having no or only short tails in the 77ish region result in low points:
http://prntscr.com/7qumi1
(low scorer)
whereas if we have some ample tail there, the points are higher. I wonder if this tail acts as a release to the miRNA, such as in this case:
http://prntscr.com/7qupck