Mongos/mongostat are reporting 3xinserts than what the shard is seeing. In this setup the inserts are going to a db on just one of the shards, and the updates are being split (unevenly by the app) to two of the shards. This is with no sharded collections, they are just sharded dbs and each db is on a different shard.

http://gist.github.com/bd3329ab8502ec4cbdbb