Effectively an issue of how Direct3D works. The field of view setting refers to the horizontal dimension. The vertical one is then scaled according to the given aspect ratio. That means if your screen is wider, giving a higher aspect ratio, the same FOV setting will actually show less of the vertical FOV. As the overall max FOV value is fixed, the max vertical FOV value is thus lower than with a 4:3 aspect ratio monitor, even if the resolution is the same.
If you tilt your screen 90 degrees and set a resolution of 1200*1920, you should see the opposite effect: now your vertical field of view will be set to more than what you could achieve on a 4:3 screen. Usually though that will lead to major distortions when zoomed out ;)