I have a good memory but it's very selective, and it has its own criteria for deciding what it remembers. I wish I knew what they are
Anyway, I forget most of what I read very quickly, except for a general impression (usually) and a few snippets (sometimes). Which is why I re-read often, maybe.
As for whodunnit, I just don't care, and besides, by the time I get to the end I don't even remember who was killed