I'm sure this will spark an argument... but I want to make a few corrections. I'm not trying to pick on you... just want to put a tad more accurate info out there. Like you... my post is not very technical... more of a general concept.
You are mistaken about wider signal having more range. The opposite is true... narrow band has better range. Wide has better audio fidelity. Wide receivers have less usable sensitivity than narrow bandwidth receivers because the wider the receiver, the more it fills with the broad spectrum noise. It then takes more desired received signal to pull the data out of the noise, reducing range per watt. This is why major manufactures collectively agreed, many decades ago, that 2.5 kHz bandwidth for SSB voice transmissions would be considered "normal" bandwidth.
Also, the "bandwidth" and "channel spacing" are not the same. It looks like you may have mixed them up. In real-world application, GMRS WFM (aka wide band) "channel spacing" is 25 kHz and the typical occupied bandwidth is 20 kHz. The NFM (aka narrow band) channels are spaced 12.5 kHz and are typically 11 kHz occupied bandwidth. I measured one of my radios moments before posting this. My radio on wide band is 19.3 KHz with 4.4KHz deviation and narrow band is 10.3 KHz with 3.5 KHz deviation (a little hot).
In laymen terms, the more bandwidth you use, the more data you can send. The more data you send, the better the fidelity can be. In GMRS voice, that data is your voice. Therefore, on wide band, you can have better audio fidelity due to more information transmitted and received. The reason audio from a narrow band transmission has low audio on a wide band receiver is because the receiver is tuned in such a way that it is listening to a wider spectrum of radio than the transmitted signal, and it is expecting more deviation than provided for the voice. The empty RF space is filled with random noise (same noise that reduces sensitivity / range) and the lack of deviation leads to lower audio levels. You combine the two and it can sound like less than half the expected/normal audio levels.
The inverse is true when a narrow receiver hears a wide signal. The receiver is only hearing about half the signal (making it sound lower in tone) and the over-deviation causes a loud audio level... sometimes sounding a little distorted.