Different Key orders make the document different in MongoDB?

[mongod@localhost ~]$ mongo test

MongoDB shell version: 3.2.6

connecting to: test

> db.mycol.drop()

False

> db.mycol.insert({'_id':1,x:1,y:2,z:3})

WriteResult({ "nInserted" : 1 })

> db.mycol.insert({'_id':3,y:2,z:3,x:1})

WriteResult({ "nInserted" : 1 })

> db.mycol.find().pretty()

{ "_id" : 1, "x" : 1, "y" : 2, "z" : 3 }

{ "_id" : 3, "y" : 2, "z" : 3, "x" : 1 }

> db.mycol.createIndex({x:1,y:1,z:1},{unique:true})

{

        "ok" : 0,

        "errmsg" : "E11000 duplicate key error collection: test.mycol index: x_1_y_1_z_1 dup key: { : 1.0, : 2.0, : 3.0 }",

        "code" : 11000

}

> db.mycol.find({x:1,y:2,z:3})

{ "_id" : 1, "x" : 1, "y" : 2, "z" : 3 }

{ "_id" : 3, "y" : 2, "z" : 3, "x" : 1 }

> db.mycol.find({x:1,y:2,z:3},{x:1,y:1,z:1})

{ "_id" : 1, "x" : 1, "y" : 2, "z" : 3 }

{ "_id" : 3, "y" : 2, "z" : 3, "x" : 1 }

// some proper formatting by re-arranging the column/key names

> db.mycol.aggregate([{$match:{x:1,y:2,z:3}},{$project:{xx:"$x",yy:"$y",zz:"$z"}},{$project:{x:"$xx",y:"$yy",z:"$zz"}}])

{ "_id" : 1, "x" : 1, "y" : 2, "z" : 3 }

{ "_id" : 3, "x" : 1, "y" : 2, "z" : 3 }