It's arbitrary but considered safe by most authorities where such things are monitored, professional drivers are given a bit more leeway,

https://blog.orbcomm.com/tachograph-rules-european-drivers-h...

but when they are caught exceeding the limits they are hit very hard (possibly including confiscation of the vehicle & cargo, as well as massive fines and possibly loss of drivers license). And that's good for everybody, the lorry drivers included, it gives them the expectations of a reasonable work day as well as the improved safety for everybody else and a level playing field. Fraud with these devices used to be rampant, it is getting a bit better now with the digital generation which is more tamper proof.