Routing doesnt gaurantee that all the items on a shard will have the same routing key, it will make sure that all the docs that share a routing key are all on the same shard.
Here is a good ES post about it
And a quote from the bottom:
This shard almost certainly has documents other than those belonging to user123. Routing ensures that documents of a particular routing value all go to the same shard…but that doesn’t mean that other documents aren’t routed to the shard too.