The realization I had was that your characters are, for lack of a better word, frequently "snappish" in ways that didn't make sense to me. The problem my subconscious had was that I am used to thinking of someone who "snaps" as being at least annoyed and perhaps mad. However, you use it so often and the characters weren't always annoyed or upset, as in the above quote, that it bothered me."Yeah, the Cypress Hills cover about 2500 square kilometres. Which is what, almost 1000 square miles, I guess?" Little Jo snapped back almost instantly.
"You've been studying."
"Well, of course," she giggled.
I would like to respectfully and mildly suggest that you find some other words to use other than "snapped" when people aren't biting off their words in anger. For example, in the above quote I would have felt more comfortable with something like "Little Jo rattled off almost instantly."