After the quiz closes, you can go to results and it shows how many attempts were made by the students. For a recent quiz, the item analysis showed more attempts in each question than what the summary said. I tried all of the options to see if more "attempts" would show up in the summary page, but could not get any more to appear.
Using phpMyAdmin, I went into the quiz attempts table and saw that there were indeed more attempts than what the summary was telling, and it matched the number I was seeing in the item analysis page. However, the extra attempts belonged to instructors previewing the quiz as it was being put together (the preview element was set to 1 and the user number was for an instructor, not student) . After deleting these records in the database, moodle showed the same numbers in the summary and item analysis pages.
I noticed at least one open issue related to quiz summary not showing attempts that had a "zero", and I recall that instructor preview attempts used to show up there (with a zero score) in previous versions, but not in my current version. I remember deleting these attempts when I was on the summary page, so I never noticed the number mismatch before. I agree that preview attempts should not be listed in the summary, but they should also not be included in the item analysis.