The display of alpha-pre-multiplied pictures in "Draw Image with Alpha" mode is made as is the image was not pre-multiplied and so is incorrect.
* open the linked .blend file
* in image editor, make sure you are in "Draw Image with Alpha" mode
-> you can see that the edges of the "Render Result" are badly composed with the background of the viewer, they are dark.
Actually if the image is not premultiplied, as in "Viewer Node", composition is OK.
Note: on the contrary the display when "Draw Image with Alpha" is off is correct (but this time it is the display of the non-premul image that is wrong).