I looked up those stats. First of all, it is literacy in 'English'. A good portion of the country does not speak English at home. Second, it was assessed in 2003, and a disproportionate amount of those with 'below basic' prose literacy were over age 65 at the time. The assessment before was done in 1992 and there was an a marked increase in quantitative literacy between the two.

* https://nces.ed.gov/naal/kf_demographics.asp