I imagine Google wants to distinguish between websites that want to be blocked by SafeSearch, versus websites that want to be blocked when parental controls are on? There's no reason to leave that ambiguous. Plenty of adults have SafeSearch on.

Defining a new header isn't hard; the hard part is getting consensus and adoption.

For what it's worth this header has been around for a long time. It's predecessor (PICS ICRA) was too complicated and started using topics. After a while they added so many topics that even being an abbreviated header it was still massive and confusing. There were websites that people could select all the topics and what not but even then the adoption was low due to complexity and topics constantly changing on sites.

Are today's videos beheading's without blood, with blood, with or without anguish, with nudity, without nudity, etc... After a while it gets out of hand.

It turned out the internet was too dynamic so the RTA header was created to just say "adult".