There are 3 non-overlapping 2.4Ghz channels (1,6,11) for 802.11b, because of the channel spectrum shape. Not only "b" uses 22Mhz channels with is just a bit too wide, but also, due to the way how single-carrier PSK modulation works, especially on older hardware, "b" has considerable amount of spurious emissions adjacent to the main carrier that widen it even more - could be seen here as smaller "hills" to the left and right [1],[2].

802.1g/n/ac can easily have 4 non-overlapping channels (1,5,9,13) because (thanks to OFDM) channel spectrum is much neater with rather square 20 (40) Mhz channels with practically no energy outside [3].

Yet, everybody's stuck with 1,6,11 channel scheme which is wasting precious bandwidth. [4] (middle graph) Notice gaps between the channels that could be eliminated by moving 6->5 and 11->9, and gap on the right where channel 13 can fit after that.

[1] https://villagetelco.org/2009/11/rf-hacking/

[2] http://ecee.colorado.edu/~ecen4242/wlanb/index_files/image02...

[3] http://rfmw.em.keysight.com/rfcomms/n4010a/n4010aWLAN/online...

[4] https://www.codify.com/wp-content/uploads/2009/07/Central-Ro...