In Actual Duration column Overall duration is showing less

Hi,

I have Master schedule of Project and monthly tracking, updating project, In my project I have inserted columns: Baseline duration, Baseline start and finish, Actual duration and actual start and finsh, In actual duration column overall actual duration is showing 16.18 weeks and its sub tasks (levels) have more duration like 52 weeks, 40 weeks, 20 weeks etc, why showing less in overall? (overall is 16.18 weeks) why?

Please help me if any one know.

Thanks and Regards,

Kumar