> I don't know what it is that helped me understand these concepts, twenty years after graduating, better than I did when I was studying it everyday. But I would NOT attribute it to the textbook, given it is literally the same one.
Probably additional skills and experience you picked up all these years by solving/understanding hard problems
And more time for it to be mulled over. It was frustrating, but in college I found that the 16 week semester was rarely enough to actually learn something, only enough time to commit it to memory. It was often a year or more later that I'd have a sudden eureka moment while studying a separate topic. Perhaps something that was applying the unlearned material (like Physics I and Calculus, which were a semester apart for me since I took Chem I my first semester in college) or something related but not exactly the same (like Concrete Mathematics by Knuth et al. leading me to a realization about some aspects of calculus).
I had the experience of trying to relearn some math just a few years after first learning it in college and it was similarly much easier to learn it all a second time. I don't think I had a whole bunch of new experience. I think rather there must be some kind of residual understanding, even if it can't all be articulated cold.
Probably additional skills and experience you picked up all these years by solving/understanding hard problems