Communication protocol between pilots of aircraft and air traffic control are based on a document from ICAO published in Annex 10 Volume 2 (Communications Procedures) to the Convention on International Civil Aviation and ICAO PANS-ATM (Procedures for Air Navigation Services - Air Traffic Management) Doc. 4444.
Frequencies are a limited resource in the aviation radio spectrum as voice communications are handled between 117.975 - 137 MHz. In the busy European airspace the current and expected demand are very high for communication channels. As a result, VHF 8.33 kHz channel spacing was introduced in 1999 for airspaces above FL 245.
After this was completed, the European Commission went on to regulate airspace down to FL 195 in regulation No 1265/2007 and followed by airspace below FL 195 in regulation No 1079/2012. This came into effect Jan 1st, 2018.
From that moment on, all aircraft in European airspace will need to be equipped with these 8.33 kHz channel radio's to be compliant with the law. And practically speaking, a lot of frequencies have been changed since 2018 to comply with the new channel spacing.
With the good old 25 kHz channel spacing the COM part of the aviation band had some 760 channels to talk on at their disposal since 1972. Before that the channel width has been reduced on two occasions from 200 kHz to 100 kHz in the 1950s, and to 50 kHz in the 1960s. As radio technology progresses, better channel separation became possible. The next technology step would be to switch away from the trusty Amplitude Modulation (AM) for voice communications to a modern digital form.
Voice communications only need 5 kHz of bandwidth to be understandable. If you divide 25 by 3 result is 8.33333 and this gives enough spacing without next channel bleed. Thus the choice for 8.33 kHz was relatively simple.
With this, the number of channels increases threefold: 2280 instead of 760. And due to better electronics, adjacent channel interference can be avoided by smart channel assignment to radio stations. In practice, due to adjacent frequencies the old 25 kHz assignments some channels are not available. Hover the mouse over the image to the right to see the difference.
With classic 25 kHz radio's the number you dialed in was the exact frequency. But due to the 8.33 kHz separation, display limitation on the radio and to keep communication simple: the choice was made to introduce display 5 kHz channel steps. This resulted in the following table you see below:
Note: As you can see, the displayed frequency on your new 8.33 radio is not the actual frequency it will transmit on. Transmit frequencies are actually 8.33 kHz apart. The old 25 kHz spacing is moved up to the next 5 kHz channel but still uses the same transmit frequency.