802.11wireless-networking
Wifi channels 1, 6 and 11 do not overlap.
However, any channel in between them does.
e.g. channel 3 would use some of the frequency band of channel 1 & 6, and channel 9 would use some of the frequency band of channel 6 & 11.
Why would one choose to use channel's other than 1, 6 or 11 if that is the case.
Channel 14 has been there since 802.11b, it is the least overhang of any wireless channel which means that by selecting it, there is a low chance of interference from other devices.
However, due to wireless licensing laws, only Japan allows channel 14, in addition, channels 12&13 are not allowed in the USA. Apart from this, most countries allow 1-13.
If your router allows you to select one of these upper channels, it is most likely due to either selecting the wrong region on your router or you simply have an unlocked one.
As for devices, I am not entirely sure what is going on here - I would assume that you typed in the region on some / they are location aware and block out channels they are not allowed to connect on, and the newer devices simply see the channel of the router and as someone must of set it, they think it is ok to connect!
First of all, the U.S. only allows 11 of those 13 channels. Additionally, the original wifi developers made a mistake, of sorts, and signals within channels bleed over to their neighbors...
there are really only 3 channels you should use: 1, 6 and 11.
Keep in mind that applies to the 2.4Ghz spectrum. There's also the 5Ghz spectrum that has more channels, less contention (for now), and is supported for 802.11ac, but has less propogation through things like walls and trees. 2.4Ghz is old busted; 5Ghz is the new hotness.
That said, you can have far more than 3 devices on wifi at a time even on just the 2.4Ghz frequencies, because devices will share time on each channel. It's just like having someone listen to several conversations taking place at once in a crowded room: not everyone is talking all the time. If two people talk at the same time, the listener may have to ask one or both to repeat themselves. The more people you add to the room, the less total information you can pass around, because people will constantly interrupt one another at an increasing rate and the general background noise will start to become as loud the person next to you can easily talk. A good rule of thumb is around 25 devices per channel for casual browsing, but this can drop significantly for non-casual traffic like gaming, p2p file sharing, video streaming, and large file transfers.
In networking parlance, we say a wifi cell is unswitched and half-duplex, making it very sensitive to collisions. Wired networks typcially don't have these weaknesses (switched and full-duplex), and are also much less susceptible to random electromagnetic interference. While wifi is a "good enough" technology to use at home, serious networks or any serious application will do much better pushing as many people or as much network traffic as possible to a wired connection.
I run the campus network at a small college, and it's sad to see how many new students arrive this year who have never used a wire for network access. They think the notion of needing a wire is quaint, and don't understand the physical limitations involved, and why 80 devices (more than 2 per student on average) in dorm space the size of their parents' house doesn't work so well. Re-educating them about this is hard.
Best Answer
Cisco has a deployment page that illustrates this. The problem comes from having the center frequencies on 5kHz separation, but with 22MHz wide passbands. Normally, in a radio frequency assignment plan, you have for example a 12.5kHz passband and channels on center frequencies every 12.5kHz. Adjacent channel interference usually means you assign out every other channel in a local area, unless the spectrum starts getting crowded.
Because of the insane amount of overlap on 802.11, in a close area, say a warehouse, you can only use 1, 6, 11 without adjacent channel interference. Down the street where the signal falls off, someone else could use channels 2 & 7 simultaneously, a little further on, 3 and 8, and so forth.
As to the reason for the overlap, I'm guessing that they had too much faith in their spread-spectrum modulation scheme they were using when the specs were created.