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

There are several factors that go into this. Most of which are based on the patient's actual anatomy and their desired appearance. I would also say that some doctors have their own personal limit on how for they are comfortable moving a chin. A genioplasty is not yet a procedure commonly performed by all plastic surgeons either. Although it is becoming more popular, there is still a large gap in learning and performing the surgery itself.

The answer is that it varies based upon what is available and recommended. Too much advancement could be unstable, or, could simply look bad. A sliding genioplasty is not the same as a custom implant, or a chin implant anyway. Photos will help, for sure.