Thank you for the clarification, and I'd like to re-iterate that I do not think she's an adult. The way she talks and acts the vast majority of the time is that of a child. I also don't have a problem with age gaps in fictional stories, regardless of the legality were those involved to be real people. I just find it rather jarring that this eleven-year-old is giving what another person in these comments referred to as "schlafzimmerblick." I couldn't agree with this description more, and my imagination immediately went to why she could be doing this. Could it be a "deal with it" situation the author has created simply for the fun of it? Sure. It wouldn't be satisfying to me as an explanation, but you can't argue with word of god. Lacking that word, I thought to myself, "what is a rational explanation for this?" - and came up with my theory. Is it likely that it's wrong? Absolutely. It's just a guess on my part.
Serenata suggested that other characters have reacted to her expressions before, but I went back through and couldn't find an instance where it was clear that was what they were reacting to in particular. If anyone has a link to a page in the story where this is cleared up, please feel free to share it.