I did that 100 Science Fiction Books meme quickly yesterday, to kill off five or ten minutes before leaving work1 yesterday. I said from the outset that it wasn't accurate.2 I've been able to read since ≈1970, and I'm a voracious reader, so I've read a lot of books in my life (some of them before some of you were born) and subsequently killed off a lot of brain cells. It's perfectly understandable that I wouldn't remember all of them (there have been a few times where I've picked up what I thought was a new book, and get a few chapters into it before realizing for certain that I'd read it before).
On a related note, I've found that those books3 I enjoyed and remembered most tended to be those that I chose to read (as opposed to school/college assignments). There's just something about making an activity, even one you like, mandatory that somehow saps the joy from it. If I was assigned a book, I was far less likely to enjoy or retain4 it (although some I re-read later and enjoyed them more the second time through).
Does this make sense?
1 I worked from 6 to 3, a full day, in case anyone's thinking I slipped out early.
2 Then why do it? Ulterior motives! I've gotten some comments and recommendations from it, giving me some idea of what to read next…
3 Out of all the ones I've read, not just the skiffy.
4 After the test was taken and/or the paper was written, naturally.