The default setting automatically optimizes the value for bandwidth. If the momentary operating conditions allow, a bandwidth of 40MHz will be enabled, which is otherwise limited to 20MHz.
You also have the option of switching this mechanism off, so limiting the bandwidth to the narrower 20MHz.
The 802.11n standard specifies a channel bonding from 20MHz to 40MHz.
Telnet path:/Setup/Interfaces/WLAN/Radio-Settings/Allow-40MHz