If you limit the number of students per page in the Grader report to a number that is only slightly smaller than the total number of students, the last few students will not appear at all.

For example, If I have 27 active students on my roster and I restrict my view to 25 students per page, the last 2 students will disappear from the list (no second page available).

The trick to recreating this is testing various numbers in the "Students per page" setting. It's not necessarily straightforward because the system seems to include inactive students in the display calculation, even if they are not displayed because of the "Show only active enrollments" setting. Once you have found a number that displays all but the last 2-3 students, you should see the problem.