I think you can compare it to watching a movie. You won't recall at the end what color jacket the character was wearing, or what the name of the victim was, but you still get the movie. Even though the jacket-color might be significant in someway (like being the same color like the monster the character would become or something). It's normal and expected that a single viewing/reading will not give you all the insights.

Just like a movie, if I really like it, I could decide to read it again, and discover things I didn't get the first time around. But there is no requirement to spot every detail the first time.

I read Anna Karenina pretty fast. And the Power Broker. I'm a fast reader in general.