Operating system setting
Many systems default to 60Hz after a new connection. Select the advertised rate in display settings.
Cable and dock limits
A weak HDMI cable, hub, dock, or adapter can force lower resolution or lower refresh rate.
Power and browser throttling
Battery saver, low power mode, background tabs, and heavy pages can affect browser timing.
Mirroring and remote desktops
Mirroring to a 60Hz screen or using remote desktop can make a browser report the wrong effective rate.