Who decides or sets the difficulty level of "reading ease" (which is a sortable metadata attribute on the search page) ?

Some classiications seem a bit ...nonintuitive. For example, the Autobiography of John Stuart Mill is classified as "very diffcult" whereas "The Tempest" by Shakespeare is classified as "fairly easy".

I would classify it the other way around, but what do I know, I'm a nonnative speaker anyway.