First, upgrade to 2.3.7 if at all possible. The early 2.3.x releases had some bugs in this area. (I think we nailed the last one around 2.3.4 time.)
Now, if you are seeing time just a bit longer than an hour, it is nothing to worry about. This is for a range of reasons:
- If Moodle is running slowly when the student clicks submit, it may take some seconds (or even minutes) to process their final response. So, if the student clicks submit right at the last second (or the on-screen countdown timer submits the quiz at the last second) then the final submission may be processed and recorded a bit 'late'.
- If Moodle closes the attempt automatically for the student rather than the submission being submitted by the student (or the time) then that submission happens a bit later (the next time cron runs). How frequently is cron set to run on your system? Every 5 minutes is common. Again, all Moodle can do is record the time that it processed the quiz submission.