Within the Default AWS Dashboard the platform display both the S3 Usage Report as a chart and a Rack displaying the total S3 Bucket usage under - Storage (GB)
The figure displayed under S3 Usage in the rack isn't calculated directly using values from the S3 Usage Report, rather it's the total of the individual "Storage in GB" values returned against each bucket via the API in the S3 Bucket Asset report:
Downloading the S3 Asset Report and summing the Storage in GB column will allow you to verify the figure presented in the Default AWS Dashboard S3 Rack under Storage (GB)
This will allow you to verify the figure present in the default AWS Dashboard under Storage (GB) in the S3 rack