It's probably because unless the candidate has some external references of superior qualification (i.e. known expert) and that the case can be made to the team, then it's better to level them in at a lower level so that you don't rock the boat for your current employees. It also sets a work hard tone.

One thing the author gets right is that it's not necessarily about being brilliant but being able to work in the context of the team. And this defines the promotion ladder. This is the build trust thing. It's not specifically innovative and it leaves a lot on the table (which is why people leave.. to have a bigger impact).. but after you reach the "bar" so to speak then you will have more higher level work then you can handle.