While bandwidth is typically measured in absolute Hz difference due to the spread directly relating to the amount of information-carrying capacity, when you are

While bandwidth is typically measured in absolute Hz difference due to the spread directly relating to the amount of information-carrying capacity, when you are actually generating signals many things scale with carrier frequency. The way most of these signals are being generated is through an "information signal" (usually referred to as a modulator) being created within your device, then moved up in frequency by combining it with a "carrier signal". This separation allows the information signal's properties (such as bandwidth, modulation scheme, bitrate/Hz, etc...) to be more-or-less independent from the physical characteristics of propagation, which will be more strongly related to the carrier signal's properties (e.g. water absorption, reflection/transmission off of/through materials, etc...). However, no process is perfect, and although we would like to generate perfect signals, we can't. Distortion appears in multiple parts of this pipeline, both in the generation of the low-frequency modulator and the high-frequency carrier. Distortion typically comes from "linear" components not being perfectly linear and thereby generating harmonics of the signal passing through them. In the case of the modulator, this splatters signal energy up and down the spectrum on the order of the bandwidth of the modulator, but in the case of the carrier it does so on the order of the carrier frequency. This is all a matter of degree and depending on the application may not be that big a deal, but it definitely must be addressed for high-density communications equipment like cell networks. All transmitters have filters at multiple stages of their signal processing chains, both lowpass filters that filter the modulator before it's mixed with the carrier signal to boost it up in frequency, as well as bandpass filters that ensure the output stays within the bounds it's meant to be; but these filters only do so much and they can be expensive to create (as they can have rather fine physical tolerances) so everybody is always just playing the "do the best job we can for the least money" game. Luckily, most transmitters are also connected to antennae that provide a convenient filtering on the output (they only resonate at the frequencies they transmit at) which helps for specialized systems that only operate at a single transmit frequency, but for something like 5G is less helpful, due to the many channels it supports, causing the antenna to necessarily support a wide range of frequencies.