No I mean how can you turn down some settings on the same PC and go from 30fps to 60fps?
Because your CPU is sitting at 50% or less utilization when it is running at 30fps. It's sitting there doing nothing half of the time. For simplicity's sake, the options you are adjusting pretty much only affect the GPU. So when you make less work for the GPU, it can pump out frames faster, and since the CPU wasn't doing anything anyways, it can pump out more frames too.
That's exactly the point though: the CPU wasn't being used. If your CPU was actually being utilized 100%, you could reduce the resolution from 4k to 480p and the framerate wouldn't go up at all, your GPU would be sitting there doing nothing waiting for the CPU.
So in order to have a "60fps mode" on a console, in the "30fps mode" the developer can only really utilize half of the CPU power, because it needs to be able to run the game twice as fast.