It may happen for instance when you have partial marks on a question such that the question score can consist of a fraction above .5.
Simple scenario: one question in the quiz, 1 mark, possible partial mark 0.66. If the user scores 0.66 the quiz results will show 1/1 because it rounds the fraction for display. But the actual grade will be 66% because the grading looks at the actual float score (0.66).
hth