It's evident, for example, that drugs such as Paracetamol (Tylenol for you Americans) should be dosed by body weight in children. To make life simpler for parents, they are given age and/or weight brackets, sometimes along with upper thresholds (e.g. mg/day).
This of course means that lighter children are comparatively over-dosed and heavier children under-dosed compared to a median.
The problem is - I think this works pretty well as a safeguard against dangerous over-dosing (i.e. liver toxicity etc.).
Now how would we turn that advice into a gradual dosing recommendation? We can use mg/kg body weight as is done e.g. in antibiotics. But that carries the potentially fatal risk of miscalculation, and some parents might intentionally overdose over a wrong risk perception.
What we would need is something like an exponential risk curve, indicating a "safe zone" and a "danger zone" while highlighting some critical threshold. This again would need to be age/weight-specific.
Do we think parents would be deterred from giving a kid too high of a paracetamol dose? I'm not so sure, especially over time.
So in the end, I think that in some cases (especially with self-administered dosing) round numbers and sharp thresholds may work well to mitigate fatal risks, even while increasing nonfatal risks.