The Disk Usage metric in the Disk Usage Report (Reports > Azure Usage > Disk) represents the average Disk Hours # for the chosen interval.
# Disk Hours ÷ (total hours for the chosen interval) = Disk Usage
For example:
To calculate the numbers shown above:
Take each # Disk Hours and divide by 24 (total number of hours for the daily interval)
138.59 ÷ 24 = 5.77
136.13 ÷ 24 = 5.67
38.15 ÷ 24 = 1.59