We have other posts like that. Sorry as I am not the author or hardware desinger you have to pester the makers why this is so.
So far and it wasn't supposed to be like this, I only get the top speed when I get the N gear from the same makers.
I have about 20MB via ethernet
Atheros a928x in laptop
Problem is when router set to N only, I only get about 500kb-1mbs.
When set to G only, I get full 20MB/s. This holds true evn with the laptop right next to the router.
No interference from other routers as far as I know