Meaning of frequency and bandwidth of a signal, despite the fact that we do not know the signal
Some signals are generated by processes that are physically capable of generating frequencies only in a specific frequency range. For example, the human vocal tract can only generate signals between roughly 50 and 10,000 Hz. This does not mean that every human can achieve that range, or that there are people who can generate frequencies slightly above or below. When it is said that human voice has a certain bandwidth, it does not mean that every instance of human voice will actually cover that entire range; it only means that you will not often find frequencies outside that range.
Other signals are limited in bandwidth by design. One example is voice in analog telephony; the voice is filtered to a range of approximately 300 to 3400 Hz, and it is impossible to find frequencies outside that range in the system. Another example is analog television: the signal is designed to have a bandwidth of 6 MHz, and except for equipment malfunction, you'll never find a TV signal with frequencies outside that range.