Apart from the fact, that your data is perfectly distributed, i.e. you have two shards and each of them stores 50% of your data, the shown chunks do not match.
You say, the shard key is { 'user.group': 1, 'user.name': 1, document_name: 1 } but in the distribution stats I see {"user.group", "user.name" ,"archivename"} which is different. I don’t know your data, however I guess the number of different group and user values is rather low.