First off, bandwidth is a function of deviation and the maximum frequency of the audio used to modulate the RF signal. This is defined by Carson's rule which states:
Bandwidth = 2 x (Deviation frequency + Maximum audio frequency)
Therefore, if you assume that the maximum audio frequency is 3 kHz, which is standard for communications-grade audio, and the deviation is +/- 5 kHz, the bandwidth is:
BW = 2 (5 kHz + 3 kHz) = 16 kHz
If you reduce the deviation to +/- 2.5 kHz, the bandwidth is:
BW = 2 (2.5 kHz + 3 kHz) = 11 kHz
This is reflected in the FCC emission designator which appears in the license grant. That is, 16K0F3E is 16 kHz bandwidth frequency modulation single analog channel telephony. Likewise, 11K0F3E is 11 kHz bandwidth, yadda, yadda.
This is the technical description of the bandwidth of the transmitted signal and it has nothing to do with regulatory dictates which say the bandwidth may be no more than X or radio design decisions which say the channel step size is Y.
That said, FCC rules which require "narrowband", i.e. no more than 12.5 kHz bandwidth, do not apply to amateur radio. Consequently, most amateur radio manufacturers see no need to make provisions for "narrowband" in the radios they market to amateur radio, though some amateur radios do have a narrowband setting.
Additionally, standards for step size which apply to Land Mobile Radio (FCC Part 90) in the UHF band do not necessarily apply to LMR in the VHF band. Specifically, prior to "narrowbanding" the standard channel spacing in VHF was 15 kHz. With "narrowbanding", the standard channel spacing is 7.5 kHz while in the UHF band it was 25 kHz before and 12.5 kHz after.
A channel spacing of 7.5 kHz is not always compatible with the 5, 6.25, 10, 12.5, 15, 20, 25, 30, 50, and 100 kHz spacing that many amateur radios provide. For example, let's take the old AAR (railroad) channel 46, aka 160.800 MHz. Before "narrowbanding", the adjacent channels were 160.785 and 160.815 MHz. A radio with a 5 or 15 kHz step size will tune those channels just fine. However, now with "narrowbanding", the adjacent channels are 160.7925 and 160.8075 MHz. Most amateur radios simply do not have a step size that will tune those frequencies. They are not evenly divisible by 0.005, 0.00625, 0.010, 0.0125, 0.015, 0.020, 0.025 MHz, etc. You can get close, but you can't get right on. However, if your radio has 2.5 or 7.5 kHz as an available step size, you can tune in the exact frequency.