Good question. I can explain. "storage.googleapis.com" is an endpoint for Google Cloud Storage's "XML" API. Google Cloud Storage (GCS) is a cloud blob storage service similar to Amazon S3.

This particular API, the "XML" API, is designed to be API-compatible with S3. The XML namespace, 'http://doc.s3.amazonaws.com/2006-03-01', is therefore the same. This allows third party tools like 'boto' and the like which work with S3 to work with GCS with only a switch in hostname.