WARNING ABOUT ENABLING NOSCAN FROM WIKI:
Turning this on will violate regulatory requirements!
If you are in FCC-US land for example, this will cause your device to run out of regulatory specifications (i.e. disable scanning for other SSIDs on the channel), meaning your device is not covered under its license and is running as an illegal transmitter.
The reason you likely receive 20 MHz bandwidth is because of congested 2.4 GHz channels.
See: Linksys WRT3200ACM only transmits with 20 MHz on the AC radio
That thread discusses the 20/40 issues in more detail.