The number shown for the storage GBs in the disk usage report is the sum of the size of all disks that were active during the given time period.
Imagine 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
The default Disk Usage metric in the Azure Disk Usage Report represents the average Disk Hours # for the chosen interval.