Because '_total_bytes' is not set until after an initial request, headers returned for a new iterator will have a 'Range: 0-' header set (which is not desirable, AFAIK):

https://github.com/GoogleCloudPlatform/gcloud-python/blob/1f2aaf3606802f4f02eeb30e22e1082ef7f4594e/gcloud/storage/iterator.py#L218