There’s a little more to it than that. They also need to consider that, in cars with an analogue speed dial, the actual speed is not displayed precisely. Plus, there may be minor discrepancies between the speed dial and the actual speed, which can be affected by non-standard tyres etc. So a small “buffer” in acceptable speeds is pretty reasonable.
I recall being told by someone at a Mitsubishi factory that car manufacturers actually intentionally set the speed dials to be slightly higher speed than reality to accommodate for people’s need to break the rules just a little…!