Thinking about it, part of it is probably just built into the profile of striker he is too, isn't it?
Imagine you have two strikers who generate 1xG per game. Over 30 games both strikers are expected to score 30 goals. But if one averages 3 shots per game and the other averages 6 shots then in any given game the latter striker is both more likely to score 3+ goals (obviously) but also more likely to score 0. The average over time is the same, but the distribution is different.
With football being a low-scoring game, that makes the 6-shot striker's output less valuable. Because all else being equal you would prefer the greater chance of at least one goal in a game.
With that in mind, Nunez tops Europe's big 5 leagues for shots per 90. Which means that even if he was an average finisher, you'd expect more variance game to game compared to a player who shoots less. And if he's a worse than average finisher....
Or more simply: scoring at a good rate per game is good, but doing so while not being wildly inefficient would be better. And even if he does the former, he won't do the latter.