Better routers can help lower latency. From my experience the RT-N66U appears to help lower gaming latency compared to previous routers I have owned. Although the ping time involves latency it is in reality all relative. It depends "what you are pinging". My speedtest.net ping to the closest local area ping test server usually shows a 10ms ping. When I am gaming on my PS3 (Call of Duty and other latency sensitive online games) the ping notification in the game all depends on the "host gamer" and can vary from 50ms and up to 200ms or more.
While your router is a factor the ping time is more often the result of many other factors that are not related to your router such as the networking speed of your gaming machine, the latency of your ISP connection and how many gamers are connected to the gaming servers or host gamer at one time. In general the farther away the host gamer is the greater the in game latency and the higher the ping time or the closer that in game host is the lower the latency and lower the ping. Therefore the differences for your game ping times that you see probably have less to do with your Asus router and much more to do with the pathway your connections take to go through the various internet servers to get to the game server or the game host.
Note: If you feel your router is adding to latency of your gaming in the Asus router settings then try this.... In the Asus router settings assign your gaming machine a static LAN IP address... Then you can enable port triggering or port forwarding for the ports that your particular game uses.... or if you you feel your gaming machine is secure enough to expose to the internet (Linux based or a gaming console) you can simply put your gaming machine's static LAN IP that you assigned (PS3, XBox, etc...) in the router's DMZ.