Hello, I am also seeing this issue. I can reproduce it reliably on a Dell Inspiron 15" 7537
Windows 8, Intel 4400, Nvidia 750M, native laptop resolution is 1080p
Nvidia drivers : 331.82-notebook-win8-win7-64bit-international-whql
Intel drivers : Win64_15338
In the game(s) Bioshock Infinite and Battlefield 4, if I set the resolution in game to 720p, the image will not stretch to fill the screen up, it will be centered with black bars all around.
I have found a work around to force correct stretching of the image: I have to set the desktop resolution to 1600x900, and launch the game. I can then change the in-game resolution to anything and it will fill the screen correctly. Once I revert the desktop resolution to 1080p, games will stop stretching correctly.