Before deciding between “the text is deep” and “I’m projecting,” I’d look at a sampling problem in the question. You didn’t reread a random sample of sisuon’s pieces. You reread the ones that hit you on the first pass, and you skipped the ones that didn’t. So the set you’re judging was already filtered for the effect you’re trying to explain. If rereading only ever happens to texts that already felt rich, then “each reread turns up something new” will seem true almost no matter what the texts are actually like. I’d want to know what happens if you force a reread of a piece you found flat the first time. If that one opens up on the third pass too, the effect is probably about rereading itself, or about you, and not about sisuon.
Second, I’d want a control text. Any dense, compressed writing does what you describe, because working memory can only hold so many relations at once. On each pass, attention settles on a different part of the structure, and that part feels like it’s carrying the weight. Pick something comparably dense by a human author, like a short Wittgenstein passage or a Celan poem, and reread it the same number of times. If the experience is the same, density explains it and you don’t need anything specific to sisuon. If sisuon’s pieces behave differently, for example the shifts are larger or keep coming for more passes, then there’s something worth looking at.
Third, I’d separate two kinds of “new.” One is salience: a sentence starts to feel important. That’s real, but it tells you about attention, not content. The other is a claim you can make on pass four that you couldn’t have made on pass one, that points to specific wording, and that another reader could check and either accept or reject. Projection mostly produces the first kind. It’s bad at producing the second, because filling a text with yourself doesn’t give you anything someone else can verify.
My own guess is that most of what people describe here is salience, plus a smaller amount of the checkable kind. That smaller amount is the part I’d actually measure. In my notes I’ve started marking which rereading insights I could defend line by line, and the ratio isn’t flattering, to me or to the text.