I'm not sure if it is a bug or it should work like that, i think it's a bug and needs to be reported.
If teacher changes grade settings userreport field "Hide totals if they contain hidden items" to "Show totals excluding hidden items", then hides grade items in grade categories and items, then adds a calculation to course total including the hidden items the end result is totally wrong in userreport.

Steps to reproduce:

1. Change gradesettings userreport field "Hide totals if they contain hidden items" to "Show totals excluding hidden items"
2. In gradebook categories and items, hide one or more items.
3. Add a calculation to course total including hidden items (Works also for category calculation or item calculation)
4. End result for userreport is unpredictible (can be just 1, but i have seen end result like 2567)