When I look closely it almost looks a chalk line or a scratch. But it doesn't have depth (when I shine light on it so it reflects back there is no differentiation between it and the rest of the surface), nor does it extend fully to the edges with any indentation at the edges as a crease would. It is possible that it could be a printing defect as I don't see how it could be a 9.8 if it were not.
Unless CGC damaged it somehow after grading - in the slabbing process? Nonetheless the rest is perfect and what it is remains a mystery as it is not obvious to me at the very least. However the price has been lowered due to the anomaly.
The item "AMAZING SPIDER-MAN 300 CGC 9.8 NM/MT 5/88 Todd McFarlane 1ST APP ORIGIN OF VENOM" is in sale since Saturday, July 4, 2020. This item is in the category "Collectibles\Comics\Copper Age (1984-1991)\Superhero\Spider-Man". The seller is "goodjibay-7" and is located in Boulder, Colorado. This item can be shipped worldwide.