Because what you call 'granularity' is intentional inconsistency.
Apple is making a fairly strict policy decision (open internet == 17+). But with separated parental controls for safari and youtube, they're are exempting themselves from the downsides of this policy (equivocating Safari with apps that actually do contain vulgar, suggestive and/or violent content).
If they were at least willing to eat their own dog food, one could argue that Apple doesn't feel their policy is too strict. That they don't demonstrates that even they can see that the classification system doesn't quite work.
