It's not a question of "Can you read the book?" It is a question of, "Did you extract information from the book? Can you connect the dots asked in the question based on the information that you read?" Lots of people who identify themselves as literate have a lot of difficulty doing these kinds of things.
I'm really sorry if this comes across as a TL;DR, but there's a name for that. I'm positive you already know, but for the benefit of those interested, it's called "functional illiteracy." And it's wild, still blows my mind to this day. Like, if you're functionally illiterate, that doesn't mean you don't know how to read...it means you can read but can't understand language written beyond the basic level. There are a lot of variables involved and I'm oversimplying a lot, but that's it in a nutshell. It's fucking terrifying, to be honest, especially because it's so widespread.
Read to your kids, folks! And talk to them about it afterwards!