This is because how we store data. For example each sorted set element is stored as document.
For example
db.objects.find({_key:"topics:tid"});
{ "_id" : ObjectId("5547aee465190fe212295e96"), "_key" : "topics:tid", "value" : "4392", "score" : 1430511213134 }
{ "_id" : ObjectId("5547aee465190fe212295e94"), "_key" : "topics:tid", "value" : "4391", "score" : 1430500193058 }
{ "_id" : ObjectId("5547aee465190fe212295e92"), "_key" : "topics:tid", "value" : "4390", "score" : 1430494440461 }
{ "_id" : ObjectId("5547aee465190fe212295e90"), "_key" : "topics:tid", "value" : "4389", "score" : 1430484949083 }
{ "_id" : ObjectId("5547aee465190fe212295e8f"), "_key" : "topics:tid", "value" : "4388", "score" : 1430479608551 }
{ "_id" : ObjectId("5547aee465190fe212295e8e"), "_key" : "topics:tid", "value" : "4387", "score" : 1430435412827 }
When you post a topic it's topic id and data is stored in many places. For example under topics:recent, topics:tid, user:1:tids and so on.
