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Deviation (FM Deviation).
Is the amount of bandwidth that your modulated signal occupies either side of the transmitted (centre) frequency. Here are we can see the space occupied by a 5kHz deviated signal, as used by a radio with 25kHz channel spacing:
We can see that the transmitted information occupies 10kHz (5kHz either side of the centre frequency).
Now lets look at the 2.5khz deviation as used in transceivers with 12.5kHz channel spacing:
The signal now occupies only 5 kHz,thus allowing twice as many channels in the same amount of frequency spectrum as previously.
Now that the 144 - 146Mhz (2 Meter) band has been restructured with 12.5kHz channel spacing and the deviation lowered to 2.5kHz so that twice as many channels fit in the same space (actioned due to other radio spectrum users complaining that Hams were wasteful of spectrum because of the space being used), older radios (and newer ones that have been improperly? designed) that only have 5kHz deviation can cause and receive adjacent channel interference with the newer equipment. Just to confuse the issue of older equipment, some of the radios have a 12.5kHz step size, thus conforming to the channel 'size' but not the narrower filtering or lower deviation level.
Some repeaters and internet gateways 'reject' or ignore the 5kHz signal because over deviation makes the transmitted signal very rough and, if narrow filters are used in the receiver (as is usually the case), the edges are clipped off, so the repeater / gateway thinks it is hearing noise and won't latch 'on'.
Using an older type radio that was designed for the 5kHz deviation to listen to a radio using 2.5kHz deviation, you would need to turn the volume up a bit as the signal is ½ what the detector is 'looking for'.