It might be that your ASUS is combining two things into one UI for convenience. Mapping one IPv6 prefix on the WAN to another on the LAN is one thing. Allowing traffic to a particular IP:port to cross from WAN to LAN is another. They're independent, though of course both are needed for the overall high-level task of "allow my LAN server to be reachable to the internet", so it makes sense to have a UI that does both things under the hood.
Why? This is exactly how I want it to work: pass through only the ports (or all the ports) I want/need for the service in question.