This is such a huge percentage that it has to be incorrect, right? Over half of American adults can't really read? Or am I just vastly underestimating a '6th grade level'.
No shade at putting fingers under the words as they go, sometimes with some fonts and especially fine print it can be easy for your eye to jump up or down a line.
I'd wager it's a solid mix of both. A 6th grade level is probably marginally higher than you're expecting it to be. However, it gets much, much worse than you'd expect in a large portion of the US.
6th grade isn't bad at all. It is about the same level of reading as a TV sitcom. A person with a 6th grade reading level may be limited in regards to higher education, but reading won't be an issue in their day to day life or even most career paths. If you aren't challenging yourself by seeking good media and active reading, it is pretty likely that you will fall to around that level.