In co-op, Nova’s 2nd mastery is “Nova Primarily Ability Improvement” is listed as +1.33% per level (+50% at level 30). However, in game, snipe is 300 (+100 or 33%) damage. Same issue (+33% damage/block) on shotgun ability, shield ability, and sabotage mine.
Based on how the values are all consistently wrong it looks like the “Ability Improvement” is actually about +1.11% per level (for a total of 33%) instead of the listed/shown 1.66% (which would be a total of +50%).
These are all the listed text values and (as far as I can tell) the listed values match the damage actually being done so it’s not just an “on ability” typo where the ability is doing the proper damage.
EDIT: it’s because the listed values are including the mastery bonus. Snipe is (200 + 100) or properly 50%. I misread it as being 300 + 100.