Is this (the effect sat the edges) what you are observing?:
View attachment 412672
If so, I think it comes from the light both reflecting and essentially refracting off the edges of the carriers.
I came to the conclusion that the effect was due to the thickness of the carrier material, and the fact that the carrier material was reflective.
I happened to like that effect when I had my D6, and would look to use it when I could.
Sadly, a downsize meant the D6 had to go to a new home, and I can't reproduce the effect any more on my LPL 7700 with its much thinner negative carriers.