From the Github description, a quote: "In a class I taught at Berkeley, I did an experiment where I wrote a simple little program that would let people type either “f” or “d” and would predict which key they were going to push next. It’s actually very easy to write a program that will make the right prediction about 70% of the time. Most people don’t really know how to type randomly. They’ll have too many alternation…
Right; if you guess completely randomly it will hover around 50%, but if you choose the least-likely 5-gram from your history, then it should end up less than 50%.