[Samekas]

In short: Kodi renders in OpenGL, which is only "stopped" by swapBuffers. If this is not vsynced, it will do whatever the hw wants. Syncing to Display is btw. implemented as syncing to "vsync" which should match the repaint of the display. But won't work without vsync obviously.

If you adjust refreshrate ...

Okay, but with that disabled and only "Adjust display refresh rate" working, if the screen is at 23.976 Hz and the video file is 23.976 fps, then aren't all the frames synced? That's why I said about v-sync being disabled, it's like G-Sync or Freesync, the FPS are matching the screen refresh rate.  

 Let me ask you differently: How do you render 300 fps on a 60 hz Display? 

 Where does this "300 fps" comes from?