Because there was substantial overlap in the algebraic techniques utilized in the different subjects I was studying, my exposure to them per day was higher, so that when I learned them, they stuck in my long-term memory.

Counterpoint: this paper seems to indicate that this sort of "overlearning" doesn't work:

As shown in Figure 1, overlearning provided noticeable gains at 1 week, but these gains were almost undetectable after 4 weeks.

One thing I've often wondered: it seems like the people who like math the best are often also the people who are really good at it. There don't seem to be many people who are bad at math who like it. I wonder if that's because the way math is taught in school, if you're not one of the top few kids in the class, part of your experience in the class is developing an identity as a person who isn't the best in the world at math and feeling intimidated by those who are really good. Perhaps the fact that you were just auditing classes, or the fact that you were self-studying, allowed you to escape this identity and thus grow to like math.