It's just measuring adherence to a particular standard of written english. To use it to determine whether one work is "better" or not, you need to make a value judgement about which written standard or dialect you want to consider the target and compare to.

Choosing that target is the ideology mentioned in an earlier comment: holding one dialect as superior or more fit to a purpose is ideological in a pure technical sense.

You could also choose a different standard to get different results, for example imagine telling it that this is the goal for "best" https://www.bbc.com/pidgin

Anyway since it mentioned using wikipedia as a source for "good" english, it makes sense to me to intuit it that way. Prose on wikipedia is detailed and held to a very specific standard, which is legible to the filter. Prose on wikipedia is also not usually going to win literary awards though, since that depends on more than careful adherence to a standard.