So your speedo reads 100 km/h in a 100km/h hour zone. The intention is that you just treat that as a sign that you're at the limit and don't go faster.

Yes, you _could_ do some mental math and figure out that your speedometer is probably calibrated with some buffer room on the side of overreporting your speed, so you're probably actually doing 96km/h and you know you probably won't get dinged if you're dong 105km/h so you "know" you can probably do 110km/h per your speedometer when the sign is 100km/h.

Or you could just not. And that's the intention. The buffers are in there to give people space for mistakes, not as something to rely on to eke 10% more speed out of. And if you start to rely on that buffer and get caught on it, that's on you.