Maturity value of Rs.5000 NSC post-office scheme certificate?

The advertised tax return is 8% but on my 6-year maturity certificate it is only written Rs.8005 on maturity. It was originally printed as Rs.10,005 but they have striked it and written Rs.8005. Why is it so low?
I still don’t get it. If it is 8% compunded half yearly for 6 years then it should be
5000 * (1.08)^12 = 12590
isn’t it?!!