Talk about NVIDIA G-SYNC, a variable refresh rate (VRR) technology. G-SYNC eliminates stutters, tearing, and reduces input lag. List of G-SYNC Monitors.
I just saw your new comment. I have the MSI afterburner OSD, it shows 142 FPS. But the actual monitor's OSD shows 120. It also shows "120hz Gsync Racing mode".
If the monitor OSD says 120Hz, then that game is probably not true fullscreen but rather borderless?