The amount shown for Storage GBs in the Disk Usage report is the sum of the size of all active disks during the given time period.

Example

3 Disks, with the following sizes and active days within a week:
 
Disk1: 100GB, days 1-3
Disk2: 200GB, days 3-5
Disk3: 150GB, days 4-7
 
The weekly report would show 450 (i.e. 100 + 200 + 150)
 
The daily report would show the following values for the 7 days of the week:
 
Day 1: 100GB
Day 2: 100GB
Day 3: 300GB
Day 4: 350GB
Day 5: 350GB
Day 6: 150GB
Day 7: 150GB