This is a CRITICAL bug.
different score values are shown in singleview and grader report, and the
finalscore field in grade_grades fails to be updated properly.

Create an assignment activity CC
Do no grading in the assignment activity
Go into singleview for grade item CC
select one student and click the override checkbox
put in a score (lets say 99)
click save
if you look in singleview and grader report the value will be 99 (this is correct).

Now
in singleview for grade item CC , uncheck the override checkbox
click save
In singleview, you will see no score (this is correct).
In the grader report you will see 99 (this is the bug).

I have a pretty good idea on the cause of the bug.
The rawgrade in the grade_grades table has a value of NULL. I believe
what happens on an "unoverride" is that it will attempt to update the finalgrade
from the rawgrade field, but if the rawgrade field is null, it either fails or doesn't do it. If there was grading done in the assignment activity, the raw grade would hold a value, and when the "unoverride" is done, it would work properly.

A secondary issue is, why does single view show the null grade.
I believe the answer on that, is there is some additional logic that says -
If the grade type is an activity, and the override is clear/not set, then display the grade in the rawgrade field (rather than the final grade field).

I'm not sure if this is what should be happening.