Spanish-speaking immigrants are often also illiterate in Spanish. It's a big challenge for educators. My kids are bilingual (not in Spanish) and their teachers perceived that as a problem. It wasn't.

And there are other immigrants who are better educated and more fluent in English than 99% of Americans.

And there are degrees of literacy. All of that 21% are not fully illiterate, they're below a standard that this institute defined. Still shameful, but not catastrophic.