IFS and IA&AS are ahead in promotion ?

Officers of which service get early promotion ? Certainly it is not IAS officers. Surprisingly it is IFS and  IA&AS officers who get fast promotion. According to information IFS officers were promoted to the rank of Secretary one month before the IAS empanelment  similarly  2002 batch IA&AS officers had been promoted to SAG one month before than the IAS officers .IAS officers of 2001 batch have been promoted to SAG(JS) on July 30.