Hi Tomas,
May be you can post updated screenshots? It would be easier to visualise or may be find the issue.
P.s. I am kind of confused that you do not have Max Grade column in your Categories and Items screenshot. If you click on the Full view link - will it show?
And when I look at the User repot, it shows the possible range for grade items as 5 (min) to 1(max). Are you using Scales?