It's 2 different models the G-Sync version is 100hz with Nvidia GPU and 60hz with AMD. The Freesync version is 75hz with AMD GPU and 75hz with Nvidia GPU.
This is because the X34 display is native 60hz that is being overclocked using G-Sync / Freesync technology. Since the G-Sync requires a module in the monitor itself it is able to achieve higher refresh rates than Freesync, but the module is designed by NVIDIA and they don't support AMD GPUs and therefore run at the default 60hz.