strange, i can only suggest your screen my have been in 60hz mode and not switching to 120hz because it`s even noticeable the first time you move your mouse or move some windows around.I`ve sometimes had games that leave the screen in 60 hz mode despite selecting 1920x1080.Also i assume your graphics card had the power to play stuff at 120 fps.
One thing 120hz screen`s most certainly are NOT,is the emperors new clothes.