They tend to have the same level/track/title/whatever-your-company-calls-it, but produce more output in the same job. Sometimes, they produce so much more output or produce output that none of their peers could produce, that it's as if they have a different job.

Said another way, these are the people that I cannot help but think of when I read Joel's essay on hitting the high notes: http://www.joelonsoftware.com/articles/HighNotes.html