Making Generalizations about "better"
Dad: Why are they better?
Z: I don't know.
Dad: Well, in what way are they different?
Z: Oh! Other publishers have maybe an illustration at the start of each chapter, but this publisher has many illustrations inside the chapters.
I think it is interesting . He explained why he liked one better (i.e. why they were better for him), but he could not say why they were better as such.
When kids are speaking and starting to reason, but still in kindergarten or first grade, they're prone to hasty generalization. I think the conversation above reflects a (third grader's) new unspoken caution in drawing generalizations. He's clear about his knowledge within his own context, but is not assuming that to be necessarily applicable to a wider context.