Didn't we learn from the Flashback printing plates that the black borders had their own individual printing plate, separate from the other parts of the image?
That's exactly what this is. There's a 5th unit printing just the black border and it's very weak. If there was only one black plate for the artwork as well you would see that the artwork is weak with black as well, and it looks pretty close. The reason the other side doesn't show any difference is because the other side of the sheet is running on separate press units, so what happens on the top of the web doesn't affect what happens on the bottom, and visa versa.