Problem with Original Duration

I calculated original duration manually, it shows 22 days while in Primavera it is showing 24 days, 6 hrs. I have checked project, activities and resource calendars there I have maintained 6 days & 8 h/d working.
 

When I change activity duration, say 4 d => 3d 8 h, original duration does not change. But when I change activity duration, 4 days => 3 7 h, original duration changes to 24d, 5 hr.

Here I realized every day (in 22 days) is having 1 hr extra (other than 8 hrs) in calculation so it is giving (22+22*1/8 ) D = 24 D, 6Hrs but I do not know why/how is this 1 hr getting added? Please mention solution, if anybody knows