Earlier quoted context omitted.
There's certainly more than just the number confusion, but I specifically remember the case study we were looking at included a survey where a sizeable number of respondents said 1/4 was bigger than 1/3.
I don't know anything about this particular one, but just because it's a case study in a textbook doesn't mean it's true. There are lots of untruths that get propagated long after they've been debunked.
The larger point is that it does demonstrate what the actual data shows - that the US has a sizable portion not good at math/fractions or finance.