When a course has the Hide totals if they contain hidden items set to Show totals excluding hidden items and the hidden assignment has a grade, the totals in the User Report are different from the Grader Report. The only way I can find to make them the same is to clear any grades from the assignment. Is it possible for the Grader Report to also hide totals that contain hidden items? This was a problem for us because a student saw a different grade from the teacher's grade.
Here is a pic of the grader report: note the 32 in the category total.
Here is a pic of the user report - same student - and it shows a 64 in category total.
How do I get the two reports to calculate the same totals?
There are a couple of trackers open on this. See here and follow the web of other issues attached/related to it.
As to an explanation, I remember reading on a Moodle Doc that gave an explanation. At this time I haven't been able to locate it though. The simple answer is that teachers will always see all grades; regardless if they are hidden. This way they have the most up to date in the gradebook. If the tracker is solved in the future, that would give you the behavior you desire though. So at this time, I would vote for the tracker to help push it up the development list.