SharePoint keeps a record of the total storage consumed by each Site collection. This number is used by SharePoint to determine if a Site collection is at risk of exceeding the storage quota.
It is possible that this number is incorrect and may not accurately reflect the total amount of storage that is being consumed by the Site collection.
The following PowerShell command can correct this limitation:
Note: Replace the URL in bold with that of the Site collection that you want to calculate the storage for. You can run the command manually or save as a .ps1 file and execute it as a script.