Even if one hasn't played a piece, it is relatively easy to put on a scale of 1-10, using a comparison technically to other pieces that you have played that are at a certain level, or to place it sometimes, by just looking at it.
I would have put Clair De Lune, by Debussy at about a grade 6 AMEB (Australian Music Examinations Board) level. And apparently it is.
Most of the grades of the pieces I've played on the list seem fairly accurate to me. Though some seem a little more difficult than I would have rated them at. Mayeb I just don't think myself playing at a grade 7 level, which is why I don't believe that some pieces are grade 7 level that I know.