In that case, the entire organism is forming. It can use techniques like forming the general structure before filling in the details. It could be that forming t

In that case, the entire organism is forming. It can use techniques like forming the general structure before filling in the details. It could be that forming the general structure is carefully scripted by chemistry, while the details are mostly position-independent. Bones can have their lengths determined by biological clocks that operate over years. Examples of clocks are hormone release cycles, which the rest of the body uses for signaling. But, a new leg wouldn't have access to the global clock, since it would've shut off if the affected parts are fully-developed. Bones and muscles might have their final shapes partially determined by outside forces. The biggest one is gravity. It may be that people develop longer legs in space, or their spines don't develop correctly. A leg intended to be regrown on an adult might not have time to be shaped in this manner.