Many, many wire protocols use 5 bits of bandwidth to send 4 bits of information, for various reasons. So dividing by 10 gives you a better estimate.
Of course when gigabit became a thing, your practical throughput was more like 75 MBps for a very long time, and being off by 25% in capacity planning is a pretty big error (one I've seen numerous engineers make, and a few make both, which means you're off by 40%)
