Hi Sebastian,
There is a difference between the Mobile Screen Size and Monitor Resolution:
Monitor Resolution: The width and height in pixels of the entire monitor, regardless of browser size. It is calculated using the javascript methods screen.width and screen.height.
Screen Size: Used only in mobile reporting. It is based on the mobile device's user-agent string, where it references a lookup table that contains screen size for each device.
Though these two reports display in essence the same thing, they are also mutually exclusive from each other. Monitor Resolutions report exclusively for non-mobile devices, and Screen Size report exclusively for mobile devices.
I hope this helps.
Thanks