The wireless N device will connect at 130Mbps but the transmission rate will be limited to a/b speeds if there a/b devices transmitting on the network. If your a/b device transmits often the N device will operate as an a/b device.
So if you are connecting something that doesen't send data constantly or only connects periodically (iPod touch say) you will see N speeds most of the time. If you are connecting a windows machine as a/b the netbios requests alone will degrade the network to a/b speeds.