This can even happen long after you've done an initial pass through the material. A few years after college I reviewed some electrical engineer concepts that had simply never made sense to me, but this time around everything suddenly clicked into place.
It's so strange, you read the material and sometimes you either struggle with understanding or even don't see how it's relevant and not just a waste of time.
Then when you review it much later, it's like opening a new room of knowledge, it just makes sense and it makes me so happy in a strange way.
I've had this happen often with music theory. Music theory it pretty dull on it's own and small gaps in knowledge excludes you from understanding a lot more.
Relatable. Presumably what takes it to click is the original knowledge, as presented in the book, plus the experience one happens to accumulate over time, even unrelated to the original knowledge. In other words, the author expected for the reader to click based on the pure knowledge alone, which has since been proven inadequate. So, in a sense, it's the author's fault.
PS: electrical engineering has one of the most reader-hostile presentations, so all authors should be tried for crimes against humanity /s
