Functions score query returns same score

Elastic Stack Elasticsearch
1

I am using functions score with sampler aggregation to match last visited documents.

ES Query

{
  "query": {
"function_score": {
  "boost_mode": "replace", // we need to replace document score with the result of the functions,
  "query": {
  },
  "functions": [
    {
      "field_value_factor": { // return `lastvisited` value as score
        "field": "visited_time"
      }
      ,"weight":1
    }
  ]
}
  },
  "size": 10000
}

Response

{
  "took" : 5,
  "timed_out" : false,
  "_shards" : {
"total" : 2,
"successful" : 2,
"failed" : 0
  },
  "hits" : {
"total" : 5,
"max_score" : 1.45973969E12,
"hits" : [ {
  "_index" : "idx0",
  "_type" : "8001",
  "_id" : "null-100-1459739724631",
  "_score" : 1.45973969E12,
  "_routing" : "100",
  "_source" : {
    "visited_time" : 1459739724636
  }
}, {
  "_index" : "idx0",
  "_type" : "8001",
  "_id" : "null-101-1459708570522",
  "_score" : 1.45970862E12,
  "_routing" : "101",
  "_source" : {
    "visited_time" : 1459708570525
  }
}, {
  "_index" : "idx0",
  "_type" : "8001",
  "_id" : "null-101-1459708599619",
  "_score" : 1.45970862E12,
  "_routing" : "101",
  "_source" : {
    "visited_time" : 1459708599620
  }
}, {
  "_index" : "idx0",
  "_type" : "8001",
  "_id" : "null-100-1459708476386",
  "_score" : 1.45970849E12,
  "_routing" : "100",
  "_source" : {
    "visited_time" : 1459708476387
  }
}, {
  "_index" : "idx0",
  "_type" : "8001",
  "_id" : "null-100-1459708421417",
  "_score" : 1.45970836E12,
  "_routing" : "100",
  "_source" : {
    "visited_time" : 1459708421492
  }
} ]
  }
}

I don't know why it returns same document score?