There are many other factors in the communications channel that can reduce the transmission speed from the maximum. That's what the high pitched squeals and squawks are that you can hear when a modem is trying to connect. Well, maybe we can find a common language so these guys can get connected. A protocol defines what information is exchanged and in what order. So you'll see things like v.25bis, v.34, and v.120.
Some protocols are more stable or faster than others at certain tasks.