If you have any global increased damage in there, then it’s added to both lines, but when the damage is calculated, it only includes it once. I’m assuming the 260% in your first one is global so if you ignore the +260% poison damage & just use the 316% then you get the correct figure = 20 * (1 + 3.16) = 83.2 (rounded down). It works the same for the others as well.
So this isn’t a bug per se, just an unclear character screen, which is why we need a good skill/tooltip dps.