How is it possible for people to think they did horrible and then get super good grades?

I studied so much over the semester that i’m REALLY hoping this is me

BUT

the thing is I remember every question on the exam and wrote them down and I know EXACTLY where I messed up. you know? so I can’t imagine that i’ll do better then I thought because that would mean everyone would have had to botched those same questions and I don’t really see that happening.

I’ve heard a lot of people say they thought they failed but then got an A and i’m really confused by that. I mean that happened to me on the LSAT (I truly don’t understand how I did so much better than I thought) and hoping that happens again but how?

why would you think you did bad, wouldn’t you have to know the answers and what you missed, ect?