That caught my eye too, and I guessed on Twitter that it might be a typo/under-explained.
Somebody from Tumblr (though on a different team) was nice enough to respond and guessed that it was because all media on S3 was not counted.
https://twitter.com/#!/adamlaiacano/status/16914079173274009...
Thanks. It seemed completely unreasonable to me until I saw this explanation.