Gradebook Sum of grades shows wrong total if items are hidden

When using the Sum of grades aggregation, if one of more items are hidden the total shown to students is no longer the correct Sum and appears to be the 'Mean of grades'

Steps to reproduce:
Make 4 Assignments (or just grade items), set their max grades to 10, 20, 30 and 40 (total = 100 to make it easy)
Find a student and mark them for each so their scores are 5/10, 10/20, 15/30, 20/40 (50% for all)

Set the Aggregation in Grades to 'Sum of grades'
('Extra credit' and 'Aggregate only non-empty grades' all unticked)
View User report and you'll see a total of 50/100 for the student (or login with the student account for the same view)

Asmt1 5.00 0-10
Asmt2 10.00 0-20
Asmt3 15.00 0-30
Asmt4 20.00 0-40
Total 50.00 0-100

Go to "Full view' and hide assignment 4 (I used with the 'eye' in the Actions list but other methods produce the same result)
Go to 'Course grade settings',
Change 'Hide totals if they contain hidden items' to 'Show totals excluding hidden items'
And change 'Show hidden items' to 'Do not show'
View User report
The 3 remaining items are 5, 10, 15 which total 30 however total shown is now 37.5/100

Asmt1 5.00 0-10
Asmt2 10.00 0-20
Asmt3 15.00 0-30
Total 37.50 0-100

Whichever item is hidden the total is always 37.5/100, if 2 items are hidden the total is 25/100.

Expecting that when an item is hidden the Sum total should not include it so when item 4 was hidden the total should be 60 and the student score 30/60 however the total remains at 100.

I've tested this in 2.3, 2.1 and 2.0 all do the same.

I couldn't find anything similar in the tracker but found this which may be related:
http://tracker.moodle.org/browse/MDL-14831