In your Nvidia control panel there is a setting call Vertical Sync or "v-sync". If this is forced on, and your monitor is a 60hz refresh rate monitor, you will never go above 60. Which in most cases is a good thing to prevent screen tearing. If it's not on in the control panel, odds are the game has it enabled by default and you just haven't changed that setting. You shouldn't need to go past 60. If your monitor is a 60hz monitor, there is no reason to. You won't see the difference.