为什么Mongodb中的aggregate（）以相反的顺序返回？

Question

Bson group = group("$NAME",sum("amount", "$amount"));

        AggregateIterable <Document> output = collection.aggregate(asList(group));

        MongoCursor<org.bson.Document> cursor = output.iterator();
        while(cursor.hasNext()){ 
            System.out.println(cursor.next());

        }
    }

上面的代码以相反的顺序返回结果