@cflury,
On the 2.4 GHz when we enable 20/40, if the channel is selected as 1 then the next channel to it would be 5. In this manner, if the channel is 6 then next channel would be 10.
For 5 GHz, if the channel is 36 then next channel would be 40. Depending upon the bandwidth we select it would increase the data rate.