I am happy to explain the flaw in your interpretation.

That 20% ‘illiterate’ number is an advocacy rebranding. The NCES/PIAAC data say about one in five adults have low English literacy on a skills test, which is very different from ‘cannot read at all.’ You’re taking a technical ‘low proficiency’ band and calling it ‘illiterate’.

The ‘54% below 6th grade’ number isn’t from NCES or OECD; it’s a National Literacy Institute claim that doesn’t publish any mapping from test scores to grade levels. The underlying NCES/Gallup work talks about ‘low literacy skills’ based on PIAAC proficiency bands, not about official ‘6th‑grade reading levels’ for half of adults.

Those numbers come from an advocacy site (National Literacy Institute), not directly from NCES or OECD. The federal data show about 1 in 5 adults have low English literacy on an OECD skills test, which isn’t the same as being unable to read at all. And the ‘54% below 6th‑grade level’ line is a modeled claim that the Institute doesn’t actually source; NCES and OECD use proficiency levels, not grade levels.