Why is there such a large range of what’s considered the maximum possible projection with a sliding genioplasty?

Most surgeons seem to say 8mm is generally the limit, with occasionally so saying that it can go as far as 10mm. But then I’ve seen results from Barry Epply’s clinic of anywhere from 12mm right up to 14mm. Why is there such a difference between opinions on the max? I guess the individual aspects of a persons bone come into play. Thanks!