Thanks for the answers. I got a solution from another programmer:

kivy adjusts the size of the window to the desktop settings. There the desktop was set to 125%. The program is ok, it scales 25% up and then the borders appear. A behavior that I do not expect when explicitly specifying the window size in pixels.

I do not know how to detect or solve this. Someone outside will have this behavior too when using my program. Maybe I find out later.