I agree on the privates. I’d be interested to see these numbers with only public schools included and if that made any difference.
However, the report in the OP isn’t about affordability in general, it’s about affordability for poor and middle income families specifically. So average tuition-fees doesn’t mean as much there. If you earn $25K a year the fact that one state is $15K vs $30k is meaningless - you can’t afford it anyway.