If an app is targeted at Windows Creative Update it returns different (wrong) values for Viewport,PreferredBackBufferHeight and PreferredBackBufferWidth. An app targeted at build 10586 returns the correct value for Viewport {{X:0 Y:0 Width:1920 Height:1080 MinDepth:0 MaxDepth:1}}. An app targeted at Windows Creative Update returns {{X:0 Y:0 Width:1280 Height:960 MinDepth:0 MaxDepth:1}}
What type of project are you using? Windows, DesktopGL, UWP?
UWP . I found this while checking why my PickRay class had developed a bug after starting a new project using code from a working project.
More detail on this issue.
Anniversary and Creative targets
Viewport = {{X:0 Y:0 Width:1280 Height:960 MinDepth:0 MaxDepth:1}}
graphics.PreferredBackBufferWidth = 1280
graphics.PreferredBackBufferHeight = 96
10586 target
{{X:0 Y:0 Width:1920 Height:1080 MinDepth:0 MaxDepth:1}}
graphics.PreferredBackBufferWidth = 1920
graphics.PreferredBackBufferHeight = 1080