I think I got my answer after some R&D.
As per description of ES, it makes use of the Lucene scoring formula, which represents the relevance score of each document with a positive floating-point number known as the _score. A query clause generates a _score for each document, and the calculation of that score depends on the type of query clause.
So I created the query and run in Sense console. I observed that the documents which I am getting is according to _score value i.e. descending order of _score value. That's the reason why I am getting unordered results and not according to posted date field.
Happy Coding. 