Ya'll realize you are all looking at it from entirely the wrong angle, even backwards.
Usage is Not how much of the gpu is used but how much resources it uses. Very important distinction.
Resources are things like speed, bandwidth, vram, cores, voltages, whatever. You are using 100% of the gpu, the gpu doesn't need to use 100% of its available resources to do so.
Think of it like hammering a nail into drywall. YOU will use 100% of the muscles in your hand to hols the hammer. YOU will use 100% of your muscles in your arm to swing the hammer. You will NOT need to use 100% of the strength in your arm to do so. You'll only use a small fraction of strength, even if you are using every single muscle.
Same with a gpu, it works to the best of its ability, working at 100%, but doesn't need to use the full extent of that ability to do so.
Those other ppl with 98% usage are actually in a worse position, their gpus are working harder, using more, just to get the same results as you get using 90%. It means they only have 2% gpu resources left before getting bottlenecked. So if they have a giant explosion, lots of physX and vectors and particles, and they hit 100% usage, they just ran out of strength, and fps goes in the toilet.
If data running through the vram only uses 6Gb,and your card has 8Gb of vram, that's 75% usage. Doesn't mean the data gets pushed through any slower, it's still getting pushed through at 100% of the vrams clock speeds and timings, it just doesn't need to use the full 8Gb of vram to do so. Someone at 100% usage is using all 8Gb of vram, so what happens when they need 8.1Gb, the data is slowed down a cycle, fps plummets due to the wait. You usage could spike could go upto 7.9Gb of vram, 99% usage, and still not get capped.
You are FAR better off never coming anywhere near 100% usage, it means there are still available resources the gpu Can use when it has a need to, depending on the scene, and doesn't take a hit the same as a gpu that is resource capped and has nothing left to give.
Punch something as hard and fast as you are physically able to. Give your utmost, 100% power in that swing, that punch. =20% usage. Why so low? Because you are not using 100% of the strength in your LEGS to punch something, you are not using 100% of the strength in your other arm, only the One ARM. You might use your legs and arm to stand, to balance, but that's not the same as using all the combined power your legs and other arm have.