using partition returns unexpected result

Question

Query does not gives result as expected:

when query without using partition the query will return correct result: it will return the max view of all products.

But when partition is used, it won't return the max total view.

get product/_search
{
  "size": 0,
  "query": {
    "match_all": {}
  },
  "aggs": {
    "cars": {
      "terms": {
        "field": "type.keyword",
        "include": {
          "partition": 0,
          "num_partitions": 5
        },
        "order": {
          "productHits": "desc"
        }
      },
      "aggs": {
        "productHits": {
          "sum": {
            "field": "hits"
          }
        }
      }
    }
  }
}

expected:

"buckets" : [
        {
          "key" : "a car",
          "doc_count" : 383,
          "productHits" : {
            "value" : 209.0
          }]

actual:

"buckets" : [
        {
          "key" : "another car",
          "doc_count" : 383,
          "productHits" : {
            "value" : 9.0
          }]

a car has the highest hits of all.