I think it one point in the movie they explicitly say that there is no correlation between what an alien says and what they write. They further say that because

I think it one point in the movie they explicitly say that there is no correlation between what an alien says and what they write. They further say that because it's not a transcription of speech, the writing is not "bound by time". Utterances have a start and an end, heptapod writing ipso facto does not. It's possible too that the alien utterances are purely gestural, and don't involve language in the same sense as the logograms.