If Quizzes are on a Subpage then accessing the statistics page for the quiz is not possible.
Additionally from http://mysite.org/mod/quiz/report.php?id=xxxx&mode=overview, it isn't possible to access http://mysite.org/mod/quiz/report.php?id=xxxx&mode=statistics which is counter intuitive since the reverse route is possible.

Either:
-Subpage needs to enable the display of the menu items
-or a link to the Statistics page from the settings block of each quiz need to be implemented - this would be the best solution, since once inside a Quiz, by default the Navigation block doesn't show but the Settings block does.