Sorry I cant provide a picture, but actually I think it’s easy to reappear. As I use a named crossbow with 65 ranged skill to hit a raider in 3tile, that should be 65+15-4 for 2 tile of attenuatation -10 raider base ranged defence so that should be 76 while it’s 71.
I think the problem dont come from attenuatation that the first tile isnot counted as it seems it always works like this for both shoot bolt and quick shot.