It also creates an artificial sense of "newness". If we are just constantly grinding out the same tome it would become more stale on a psychological level. By simply renaming the tomes they are giving us a "new" reward to work toward. We me know on the surface that it's just another tome but on an unconscious level we are seeking a new and better reward. It's a little jedi mind trick that games use to keep the same ol same ol seem fresh.