Well, that trip to sheetmusicplus was interesting, stevebob! They at least explain their ratings up to 10 and those explanations seem to make pretty good sense. But, yeah, they, too, do not do a very good job of actually rating the 8-and-above pieces. Their particular example pieces (off to the right-hand side of the rating box) of their 9 and 10 rating levels seem particularly out-of-whack.
Maybe an addendum to Richard's point is that the higher difficulty ratings are necessarily subjective? Of course, that doesn't help somebody like me who's at a place to kinda-sorta be able to play some more-difficult pieces but not able to play many, many others.
FYI (or who cares?), I can handle most of the chordal structures, octave leaps, and so forth, but not the high velocities and very complex or odd rhythmic patterns of many tough pieces. I certainly do not have the "virtuosic level technical facility needed" for real level 10 pieces (though I play Brahms' second rhapsody of Op. 79 fairly well - and almost up to tempo - and that's half one of their example level 10 pieces).
Thank you both for responding,
Bruce