Average Values

Hi,
I just notices, that there is an issue with my average grade calculation. I'm using the German "Oberstufe" System which has grades from 0 (worst) to 15 (best). But there seems to be an issue with the averages. See the screenshots below for my created grading system as well as the overview.

I have a test with 3 grades. One example seems to work with the following values:

10 - 11 - 12 -> 11 ((10+11+12)/3 = 12)

The issue appears on lower grades / when a student just got a subset of grades, e.g.
8 - 8 - 0 -> 4 (should be (8+8+0)/3 = 5) 
8 - 2 - n/a -> 4 (should be (8+2)/2 = 5)

I have no idea, how the values are calculated here. So it seems like, I've misconfigured something. Do you have an idea, how to solve this?
 

Hi,

Send me a template of this (without students) to my email karla @ support.idoceo.net
and I´ll be able to give you a solution

Best regards
Karla