103 - chapter 3 - last lab answer

could anybody explain the mongo’s behavior

‘Mongos will try to minimize the number of documents checked by the shard filter. To do this, mongos will only send the documents matching the query (i.e. are returned by the index scan) to be compared against the chunk ranges.’

does it mean that mongos
1 goes to every shard to do the index scan
2 then filter the results by shard filter (unclear what for if point 1 true)

sounds strange