40MHz depends of the choosen channel and could be HT+ or HT- depending of the choice.
Read again the link.
PS. Another consideration: You must see if any other wireless network is using the same channels, because if the router finds that the noise/interference from other networks is too high it will falls back to 20MHz bandwidth ( 150Mb/s max ).
Edit: The Problem is that 40hz is not even in the list i can choose from. only full half quarter
Edit2: Firmware: DD-WRT v24-sp2 (06/14/11) std is my version in case that matters.