GPA calculation in German Grading system
I'd guess your subjects are part of modules?
Then you need to calculate and round every module on itself and afterwards you can calculate your GPA based on the modules.

Have you checked your Prüfungsordnung (examination regulations)?
In it the calculation method of your GPA is stated.

If the GPA is still unclear after checking the document, you should contact your Examination Office.
Maybe they have a system/display bug and/or some "buffer marks" (have you something like 18 credits open, i.e. registered, but not taken yet, then grading it with 4.0 in the system would yield a grade of 2.2) or something that cannot be checked or verified from outside.