yap viviane, I didn't use your preferred words and so I must be wrong. And the authors of the textbook didnt use your preferred words and so I must be reading them out of context.
Like, if someone claims the two sides of the lightbulb shines with an effect of 10 watts each, the inescapable conclusion is to say: "the lightbulb shines with an net effective effect of 20 watts". Simply saying: "the lightbulb shines with an effect of 20 watts" is somehow not getting the physics.