that doesn't make any sense in terms of performance. The GPU is using more % of usage because you're raising the graphics, so the GPU needs to do more work and so does the CPU, while your FPS are literally the same or even lower, having a low % of usage on a GPU only mean 2 things
1- The GPU is so capable of running the game at higher fps, than running it at the fps it is doing it, only requires 70% of the GPU to do so
2- a CPU bottlenecking the GPU, means that the CPU is pre-rendering everything the GPU does, but even if the GPU is able to deliver 200fps, the CPU can only deliver 100fps, so the GPU reduces the work because it can't receive more frames faster.
3-(exception) the Game Engine on current game is not optimized enough to make that CPU-GPU work at the maximum capability of both hardware pieces.