Measuring two different things. Your number: can read the words, mostly understand the words they read.

6th graders are generally literate. However, they're not necessarily picking up on nuance, or subtleties. And they will often not take into account how the sentence they just read fits into the overall context of the piece, and they likely won't question the narrator, assuming they're reliable.

You can imagine how half of adults being that bad at these things has colored political discourse.

There's different levels of literacy as well. If we take things like math literacy and functional literacy into account (things like doing your taxes or filling out government forms), something like 60% of the adult population is functionally illiterate (below a third grade level) in at least one area.

Schools in the US stopped teaching phonics.

The way they get that grade level reading comprehension is dubious. If you read the news, non-academic magazines, and most of the NYTs best seller list, you likely have been reading primarily around that level according to these tests.

Up until the mid-60s, the US did not have a standard high school educational system. Some cities had high schools, some capped out at elementary or "middle" school. A few people went to college, but the overwhelming majority didn't.

Early journalistic standards of the era sought to produce written works that could be marketed enjoyed by the largest appreciable volume of people. So the standard for writing was set at the 6th grade level. Everyone from the NYT to Random House targeted articles and books to the 6th grade level, because this was where the maximal market share of reading consumers lived.

Consequently we've produced a super-abundance of written material at the 6th grade level. If you're no longer in school and you're doing recreational reading, you're most likely consuming something designed to be read by middle-schoolers. And because this is the de facto standard, and has been for over half a century, we have this enormous backlog of material - classical novels, famous newspaper clips, screenplays, speeches, encyclopedias, commercials - all geared to this level.

People maintain their skills with practice and this is no less true of reading than any other occupation. So when you survey people - many of whom haven't touched a scientific paper or "advanced" novel in decades - as to their reading level, they consistently reproduce the skills for which the bulk of English written works are produced.