Currently, screen size data is pre-determined by the device's user agent to populate mobile reports, and as such it's currently impossible to obtain screen resolution without additional implementation as this help page details - https://helpx.adobe.com/analytics/kb/comparing-iphone-device-versions.html
This means that all iPhone screen sizes are reported as 320x480
As such, the accuracy of this report is largely brought into question, especially when Apple mobile products contribute to such a large portion of mobile traffic.
The data capture method for this report should be looked at to improve the accuracy of the data being captured without the need to implement additional eVars to capture screen sizes for specific devices.