The bandwidth we are talking about in this(TV) case is frequency. Think of an analog signal like a bell curve (big rounded hump). Now think of digital signals like spikes(they are still bell curve shaped just much, much, narrower). You can fit multiple digital signals in the same width (frequency range) that a single analog channel occupies.
When we talk about bandwidth on the HSI side we are generally talking about the percent of the channel (yes HSI has channels too) that is being used. Generally each HSI channel can handle 38mb downstream (less for upstream).