Learning without learning

I noticed the coolest thing the other day. About 6 months ago, I bought a book in Spanish. I didn't get very far reading it, I read maybe 40 pages. For some reason, I underlined every word I didn't know, with the expectation that I would look up the words later. That never happened. I remember thinking that this book was difficult for me, and, in fact, I had underlined an average of 10 words per page. I just picked up the book again last week to read, and amazingly, most of the words I had underlined I now know. I'm down from not knowing 10 words per page to 1 or 2. Now how did that happen? I've been continuously studying in Spanish, so I must have come across these words in other contexts. This is very encouraging for me.