Elasticsearch: Query String vs Simple Query String

What is the exact difference between Query String and Simple Query String? The only difference that is mentioned in the docs is that Query String is more advanced but throws an exception when a sent query cannot be parsed.

However, I've noticed that Query String behave differently even if there isn't any exception in the query string.

Here are two same queries but the one with query string returns a response and the simple query string returns zero documents. I want to know what is going on behind the curtain.

{ 
 "query": {
    "bool": {
       "must": [
        {
           "query_string": {
           "query": "Atoms for Peace",
           "default_operator": "AND"
      
        }
      }
    ]
  }
 }
}

{ 
"query": {
  "bool": {
    "must": [
      {
        "simple_query_string": {
          "query": "Atoms for Peace",
          "default_operator": "AND"
        }
      }
    ]

  }
}
}