Q: How does Azure calculate the amount of storage used for the purposes of billing?
A: Microsoft documents the storage billing details on the Azure Storage Pricing page. As this page explains, "Storage capacity is billed in units of the average daily amount of data stored, in gigabytes (GB), over a monthly period. For example, if you consistently utilized 10 GB of storage for the first half of the month and none for the second half of the month, you would be billed for your average usage of 5 GB of storage."
Basically, you're billed on the average storage usage for the month. Therefore, you can have high utilization days and low utilization days, but you'll be billed based on the average.