How does it work: FM stations with a frequency range

I notice that certain FM stations mention a single frequency while others mention a frequency range. For example, RTHK is a public broadcasting organisation in Hong Kong. It operates five FM channels. RTHK 1 is assigned a single frequency of 92.6 Mhz, but RTHK 4 is assigned a frequency range of 97.6 - 98.9 MHz.

How does it really work? I suspect that FM stations are assigned a frequency range when they need to cover a geographic region that present lots of obstacles to FM signals (e.g. a region with lots of mountains or high-rise buildings), so they need multiple transmission points that transmit at non-overlapping frequency bands inside a frequency range. But since RTHK 1 and RTHK 4 are broadcast in the exact same geographic region, why would one station only use a single frequency while the other needs a whole frequency range? And how do I know I have properly tuned into the best frequency band for RTHK 4 for my district? There seems to be no information on what frequency is broadcast by which transmitter in which district at the RTHK website. Do stations usually publish this kind of info?

Oops, it turns out that RTHK 1 is also broadcast over multiple bands in a large frequency range; it just isn't stated in an obvious manner at the RTHK website. I had to look up RTHK in the English Wikipedia to quickly find the list of frequency bands and their corresponding transmission tower locations. There goes my puzzle about some channels being broadcast over a single band and others being broadcast over multiple bands.