Porque Burton achou que funcionava melhor assim.
Ele explicou ao BuzzFeed:
We switched Emma and Olive's afflictions, or whatever you want to call them. But luckily we had Jane, the screenwriter, [and] we ran some [stuff by Ransom Riggs]... you know, the author is still alive! He was a good collaborator. We just sort of felt that having Emma floating would be a bit more poetic and it was in the spirit of the book. So it just felt for the movie it was a better way to go with it. Because often times in novels, characters get omitted, deleted, condensed and I think Ransom felt confident that whatever changes we made were still in the spirit of the book and so it all worked out very well that way. There was no big fights or tensions or controversies in terms of how that came to be.
Com as bênçãos dos autores.
A atriz de Emma pensou o mesmo, pois permitiu que o filme se destacasse do livro, mantendo-se fiel ao espírito dele:
Quando eu me sentei e pensei sobre isso, eu fiquei realmente feliz que eles me deram a oportunidade de criar um personagem totalmente novo, porque se você tentar replicar exatamente um livro tão amado como este livro, você nunca vai agradar todos. Então você pode também fazer uma adaptação do livro. "