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.

This was very surprising to me as well. Assuming that IDs are 8 bytes (they appear to be) and that the mean post is 250 bytes (which seems conservative to me) then the mean fanout is something like 1600. That's big.