Use of frequencies in the WiFi channel apart from center frequency
Have you read this page on Wikipedia: List_of_WLAN_channels ?

In particular this image:



shows that the type of Wifi (802.11b, 802.11b etc) determines how wide (how much space) a channel takes. Especially the N variant needs a wide channel with a lot of sub channels. Then in total only 3 non-overlapping channels result.

And when using the N variant with 40 MHz wide channels, only 2 remain!

The numbering of the (sub) channels is mainly historic, in the early days of Wifi the channels were much less wide, which also means lower data rates.