I think it's a very reasonable question. One is trying to cook the meat not the bone. It's only when one realises the difficulty of guessing the weight of the meat alone one realises what a rough guideline it is rather than a precise time you must do it for.
Shows it's all an approximation. Despite Heston Blumenthal, cooking is still more of an art/skill than a science. When it seems right to you, it's done.