custom frame builders all have their own different theories on fit, this is just one of them.
http://www.hampsten.com/Company/design.html
Frame Fit:
A properly fitted bicycle is centered on the stem, which we think of in terms of sizes: small (100-105mm, for frames in the 50-54cm range), medium (110-120mm, for 54.5-57cm range), and large (120-130mm, for 57.5cm and up range). A bicycle built around the proper stem length – giving correct reach - will have optimal weight distribution and handling characteristics. We believe that the cure for a poorly fitting bicycle is not a longer/shorter stem but rather a well-built frame using the correct length stem.