I contacted Customer Care and they responded by sending me in the direction of DeviceAtlas as they informed me that the mobile screen size dimension is parsed based on lookup tables set up on their end.
I've sent DeviceAtlas a message but I'm not very optimistic that they'll be able to help.
Even if the Adobe data is based on their lookup tables, surely everyone would be facing the same issue with missing screen sizes? I would have thought there'd be a single set of tables referenced by all Adobe Analytics installations (that'd possibly vary by AA library) and that DeviceAtlas wouldn't really have any direct interaction with our implementation of Analytics.