Whats the significance (if any) of wifi channels 12-14?

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!

Wikipedia has a good article here