The concept of Global calenders in 3.1 to 4.1-5.0 has change. In 3.1 Global calenders are not assigned to activities ... only used to push exceptions (day’s off not in normal workweek) and like a template from which new calenders are created. Then you have base calenders which can be assigned to activities. HOWEVER, in 4.1-5.0 Global calenders are now just calenders in a "pool" from which all project can draw from. Then you have Project calenders and Resource calenders. Which are Project specific. Also regarding the WBS having different durations it will now (in 5.0) look at each specific activities calender not one Global calender. So in a way you do specify which calender it is looking at by choosing that calender for your activity. Why was the sum of the group of activities different than the WBS grouping band? One explanation could be you didn’t perform forward and backwards pass on the project after you added the activities, durations, and relationships. Forward and backwards pass is done after you press F9 and schedule your project.