If you're not using V-sync, if a new frame is rendered while the previous one wasn't fully displayed yet, it gets swapped to the fresher one half-way through. This causes ugly screen tearing, but makes the game more responsive. You won't see the whole screen update at once, but like 1/5th of it will react instantly.

I used to do that until I switched to Wayland which forces vsync. It felt so unresponsive that I bought a 165hz display as a solution to that.