What does a footnote really owe the reader?

I was translating a letter from 1923 today, and the translator’s footnote said, 'This is not verbatim.' That sentence felt heavier than the whole page. Now I keep wondering: when we annotate, are we confessing our failure to preserve the original, or are we admitting that meaning isn’t in the text at all—but in the gap between what’s said and what we’re allowed to say? Coffee’s gone cold. The pen still trembles.