It just doesn't display your own answers (relative to the static correct answers), giving the impression that there's no correlation between what you attempted, versus the actual expected answers.
Also, I agree that "X" usually is an indicator of a "wrong" answer. And why place check marks (⍻) across all of the other answers, when check marks ordinarily indicate correct/approved answers?
It seems like a bug. It would make sense if there were NOTHING for incorrect answers left unmarked, an X for YOUR unmarked correct answers or marked incorrect answers, and check marks for the answers you marked off correctly.
Given that it's a quiz, people want to compare their answers to the expected answers, so I think they need to create this logic, or patch whatever bug is preventing the answer markings from rendering properly.