Frieren giving Fern a different answer from the one that she gave Himmel is one of those small things that make me love this series. It shows how much of an impact those ten years travelling with him (plus Heiter and Eisen) had on her.
It shows how much our lives aren't really shaped by the years that we spend doing nothing, like she did in the forest for, like, 500 years? No, what matters are the small moments.