I have not observed such obvious tool marks (on life rounds - there are Swedish 7.62x51 blanks which have similar ones and I always wondered if these are on purpose) and also not such unevenly distributed ones.
Below a cropped section of your image.
These tool marks are very uneven as stated, marked as dotted lines.
Also there is a dent visible in the case before firing (marked “X”).
Also there is more scratches and stains (arrows).
Maybe the reason for the dents is a combination of all togehter where the tool marks allow to pass gas back, that being uneven due to the uneven execution of those, also maybe influenced by the stains (material?) and then catching in the micro dents at the shoulder and allowing the gas pressure to build up there?
What puzzles me is that these toolmarks are present at all!
Can you get hold of the same cartridge without such tool marks and shoot some to check then for the dents?