that sounds like math sorcery to me

0-100% measures note accuracy. There's an option for the game to remove the notes it displays from discrete sections of the song, forcing you to memorize those parts. If that option is enabled, an extra 0-10% is added to represent the sections you've memorized. So in this case, the percentage is indicating that I have 100% note accuracy, but 9.9% (out of 10%) note memorization.

Percents just don't work the way they used to. Is the scale from 10% to 110% now, so people feel better about themselves? Once the Percent scale starts inflating like this, how will we measure inflation? ...Math is hard.