Hi Avinash,
I think system behaviour is correct. I believe that system is issuing date 11.02.2014 because of factory calendar settings. Please execute transaction code SCAL and check your Factory Calendar that you have assigned to your project. There you can check your assigned working days whether Sunday is checked/ticked or not.
Other way round to test this. Please create new factory calendar and exclude sunday from working days check box. Assign this factory calendar to project. Then put 6 days duration. There system will give you date as 12.02.2014. (Check this scenario in development client).
But wait for other experts to comment on this. It might be the case that I am wrong with my advise.
Regards,
Amit Rajoria