Tibia fracture treated non surgically?

i have broken my tibia bone on 15th oct 2010.it was non displaced tibia fracture but bones were completely broken in two pieces .doctor thought it can be treated without any surgery or any metal plate so he put my leg in cast for 3 months .now x rays shows healing in the bone but i have mild pain at the fracture site .now its 7 months completed post fracture but still there is mild pain at the fracture site .i want to know why there is mild pain when the x rays shows good healing ???