Can anyone shed some light as to why this may be happening? I've never seen this function before because I've always used the gateway IP to access the router.
Your WAN IP address is, in fact, the router's IP address. It's directly assigned to the router's "WAN" interface in the same way as 192.168.1.1 might be assigned to its "LAN" interface. (A router has an IP address for every network that it's on.) It does not belong to the rest of your LAN.
So whenever packets are sent to the address, if they match a specific NAT rule they get rewritten accordingly, but if they do not match any kind of NAT or "port forward" rule, they're just assumed to be meant for the device itself. Whether you're accessing the router's "WAN" address or its "LAN" address, you're reaching the same device with the same webserver (which is listening on all addresses).
As to why the packets don't match a "port forward" rule even though you have one in place – that's deliberate when the client and the server are both on the same network, because even if the rule did match packets in one direction, it would never have the chance to do the same for packets in the opposite direction. This part of the question has been answered many times; search the site for "hairpin" or "NAT loopback" for more detailed explanations and for workarounds to make this possible (which have various downsides).
In other words, it's not caused by something you would "turn off"; it's caused by the absence of an additional feature that you would need to turn on.
