please just accept what i'm telling you
Very Very dodgy - anyone that says that just raises my suspicion even more.
On a bog standard consumer Cable/DSL/Router the NAT traversal is performed on the route through the WAN interface (Cable/DSL modem) as SAAB has pointed out, there is no requirement for NAT on either the WIFI or LAN ports - indeed it does not make any sense to do that in any situation where there is no reason to hide addresses from devices on the same network as it would just complicate issues.
What I believe you are claiming is that on some devices there is similar to a PIX firewall between the LAN and Wifi Segments - whilst this is a possible requirement when joining two disparete networks in a corporate environment it is not something a home consumer would require and would be very expensive to implement for a reasonable priced device as you would typically find at someones home.
The cause is usually domestic CPE built to a price with cobbled-together open-source software from varying sources.
Never heard of a domestic household having custom built CPE to a price, especially when off the shelf devices are so cheap - each to their own, however this is of little relevance to the OP's question.
Please note - not all of us that post on these forums are just aviation nuts - some of us work in IT too, so please treat those professionals on here with the respect you would like to be treated with.