I have a monitor with 1920x1080 resolution for my laptop and a Surface Pro 3 with 1920x1280 resolution. I am developing a web page designed for full-screen viewing on 1920x1080 and 1920x1280 displays.
I have confirmed the settings for each display.
1920x1080 monitor (on Windows 8):
1920x1280 (Surface Pro 3) display (on Windows 10):
It seems that your Surface Pro 3 uses an operating system wide zoom factor of 150%. Due to this the browser returns width 1280 and height 853 (853 * 1.5 = 1280 and 1280 * 1.5 = 1920). Switch Windows' zoom factor to 100% in Control Panel and your browser will return width and height as expected.