It's not "language" at issue, it's "evaluation against a set of standards related to normative grammar, reading competency, and rhetorical skills," in which the primary factor is the difficulty of the standards.

I could write an English Language Arts test that most DUers who are native speakers of American English would fail, and let's be generous and say that means "< 40% correct." All I have to do is choose the criteria and set the level really high.