Don't worry, as long as you set it to Auto 20/40, it will stay at that, my computer does report 270mbps, where as if i swapped it back to 20mhz id only get 130mbps. This happens for both the 2.4 and 5ghz channels. Simply a programming error.
Though I never seem to get that elusive 300mbps... Hopefully in the next firmware update they let you set 40mhz, not auto 20/40.