It's not so much that she is the one who remembers the sky, so much as it doesn't occur to her that there's a difference between the air and sky.

Originally, I was going to end it "only Toph remembered they always touched it anyways." But I decided that it was okay for Toph, especially in third person, to have a little bit of poetry. I'm not as happy with it as I could be, but I'm happiest with it.