Why did the same text get different _score?

[watson]

**Elasticsearch version2.3.3:

Plugins installed: [head,ik]

**JVM versionopenjdk version "1.8.0_111":

**OS versionCentOS release 6.8 (Final):

---

my request:

{
  "track_scores" : "true",
  "sort" : {
        "_script" : {
            "type" : "number",
            "script" : "_score+(doc.pi.value/200.0)",
            "order" : "desc"
        }
    },
      "query" : {
        "match_phrase" : { "mediaName" : "微美食" }
    }
}'

---

expect result:

{
"_index" : "media_b",
"_type" : "weixinMedia",
"_id" : "MzA5NTgyOTUyNA==",
"_score" : 17.916512,
"_source" : {
"mediaName" : "微美食",
"pmid" : "MzA5NTgyOTUyNA==",
"mediaNameLen" : 3,
"mediaScore" : 2.2,
"pi" : 600
},
"sort" : [ 23.91651153564453 ]
}, {
"_index" : "media_b",
"_type" : "weixinMedia",
"_id" : "MjM5NzMxOTM4MQ==",
"_score" : 17.916512,
"_source" : {
"mediaName" : "微美食",
"pmid" : "MjM5NzMxOTM4MQ==",
"mediaNameLen" : 3,
"mediaScore" : 2.062,
"pi" : 506
},
"sort" : [ 22.97651153564453 ]
}, {
"_index" : "media_b",
"_type" : "weixinMedia",
"_id" : "MjM5OTg0OTcwMg==",
"_score" : 17.916512,
"_source" : {
"mediaName" : "微美食西安",
"pmid" : "MjM5OTg0OTcwMg==",
"mediaNameLen" : 5,
"mediaScore" : 2.308,
"pi" : 629
},
"sort" : [ 24.20651153564453 ]
},

---

actual result with explain:

 {
      "_index" : "media_b",
      "_type" : "weixinMedia",
      "_id" : "MjM5OTg0OTcwMg==",
      "_score" : 17.928457,
      "_source" : {
        "mediaName" : "微美食西安",
        "pmid" : "MjM5OTg0OTcwMg==",
        "mediaNameLen" : 5,
        "mediaScore" : 2.308,
        "pi" : 629
      },
      "sort" : [ 21.073457260131836 ]
    }, {
      "_index" : "media_b",
      "_type" : "weixinMedia",
      "_id" : "MzA5NTgyOTUyNA==",
      "_score" : 17.817488,
      "_source" : {
        "mediaName" : "微美食",
        "pmid" : "MzA5NTgyOTUyNA==",
        "mediaNameLen" : 3,
        "mediaScore" : 2.2,
        "pi" : 600
      },
      "sort" : [ 20.817487716674805 ]
    }, {
      "_index" : "media_b",
      "_type" : "weixinMedia",
      "_id" : "MjM5NzMxOTM4MQ==",
      "_score" : 17.928457,
      "_source" : {
        "mediaName" : "微美食",
        "pmid" : "MjM5NzMxOTM4MQ==",
        "mediaNameLen" : 3,
        "mediaScore" : 2.062,
        "pi" : 506
      },
      "sort" : [ 20.458457260131837 ]
    }, 

---

I don't know why the same word with different _score and the first one is not precise matching.Did I have some error or the plugin of ik is not precision?

thanks!