What is the difference between parentheses () and square brackets [] when enclosing real-time interjections in the Kane Chronicles?

Question

The conceit that each of the books of the Kane Chronicles is a transcription of an audio recording allows frequent interjections from current-time Sadie and Carter, commenting on the past events or on each others' narration. These interjections are always enclosed in parentheses () or [] brackets. I had not noticed any difference between the two, and indeed had not really considered the variation at all, until I came to this passage:

Our footsteps echoed in the silent tunnels. We crossed one of the subterranean rivers, then wound our way through the library quarter and the Chamber of Birds.

(Carter says I should tell you why it's called that. It's a cave full of all sorts of birds. Again—duh. [Carter, why are you banging your head against the table?])
_{The Serpent's Shadow, Chapter 6: "Amos Plays with Action Figures"}

From the same chapter I find instances where only one of the two enclosing symbol types were used.

Apparently I'd made a strong impression on the young man. No surprise. I am rather memorable.

[Oh, stop laughing, Carter.]

Of course, I had to hope my portal didn't open during an inauspicious moment. That could have caused all sorts of nasty side effects—but what's life without taking a few risks?

(Carter is shaking his head and muttering. I have no idea why.)

Clearly there is some distinction, or it would be unnecessary to use both kinds of symbols in so short a passage. What is the difference?

Answer 1

I realized the answer in the process of writing the question. Square brackets enclose statements addressed to Carter, while parentheses enclose statements addressed to the reader (or, I guess, listener, going by the conceit). For completeness, Carter follows a similar convention. These quotes are from Chapter 4, "I Consult the Pigeon of War".

The thing was, I'd gotten used to his being around. Now that he spent most of his time with the kindergartners, I missed him. He'd grown on me in an endearing, annoying way, kind of like my sister.

[Yeah, Sadie. You saw that one coming.]

I was still mad, but part of me knew Sadie was right. I'd expended a lot of magical energy the previous night and had gone to sleep really late. Maybe—just maybe—Sadie had my best interests at heart.

(I just caught her making faces at me, so maybe not.)