Law has lot of faith on C&AG and it wants to give full freedom to them, they feel CAG is best judge to decide what is beneficial for Govt Company, them may repeat same firm for n number of times they want , they  may change auditor within one year if they want, so its upto them. 
generally law respects govt institutions and gives them appropriate freedom to function to work. this seems to be reason why appointment norms are different for cag