The low score may actually be due to BotW. Once you've played that game, Skyward Sword isn't going to seem so great anymore.
Well, there is a precedent for large decreases in scores for other major franchise as well, although I think the average decrease is more 6 to 8 points, 11 points is a bit too much indeed, but I'm not sure if this is due to BoTW, Twilight Princess decreased 10 points and it wasn't because of BOTW
I guess reviews like to rate the amount of effort they put in making the remaster version better. Mario 3D All Stars scored 82, even if every Mario game in the collection scored over 92 points, that's because 3D All Stars is mostly a collection of straight ports, and this may be a turn-off for reviewers