Its the amount of time each part (CPU & GPU) takes to render a frame. youve locked it at 60FPS which is a 17ms frame time as the game shows, but at 60FPS your only just tickling your CPU loads, its able to produce its frame in 5ms and then has an idle period of 13ms because youve forced 60hz/fps.
Once you unlock VSYCN or place max GPU load the frame times will get much closer, ie for me on 1440p with spec in sig, you get typical GPU times of ~6-7-8ms (for 100-130fps or so), and the CPU time of 5ms. So the CPU only has a ms or so spare for each frame render, it is under much higher pressure to deliver the frame in time with the GPU now storming along. Once the two values match (i.e 1080p low settings), the GPU then has to wait on the CPU to deliver its calculation before the frame is rendered, i.e you have made a CPU bottlneck scenario because the GPU can smash through 1080 resolution frames very quickly.