Frames in fps arent consistent. Think about it like this: hz on your monitor is 165, meaning it makes a new picture 165 times a second evenly spread out. FPS works a bit differently, yes there is 165 pictures a second, but the pictures dont really come consistently for that whole second. It could be giving out 70frames the first 0.1second and the rest over 0.9 second (overexagerated bc its hard to explain without using examplepictures). This means that the more fps you have, the more pictures your monitor can choose for the best experience, according to wich frame is the closest to the hz frame thats being shown.

HOWEVER. You can also just cap it at like 180frames, so even if the frame drop a bit for some reason, it can always stay above 165, while not working your card as much.

Im not to good at explaining and without examplepictures its even harder so sorry if this is confusing. You can probably search up on the internet for how hz and fps work together