When elasticsearch aggregation terms setting size is different, why do I get different DOC_counts even with the same doc_key

[shencangsheng]

Elasticsearch Version

7.17

Installed Plugins

No response

Java Version

jdk8

OS Version

Linux genecast 5.4.0-153-generic x86_64 x86_64 x86_64 GNU/Linux

Problem Description

This is my aggregated DSL

{
  "aggregations": {
    "nested-gene5495": {
      "nested": {
        "path": "snv"
      },
      "aggregations": {
        "gene5495": {
          "terms": {
            "field": "snv.gene",
            "size": 10,
            "min_doc_count": 1,
            "shard_min_doc_count": 0,
            "show_term_doc_count_error": false,
            "order": [
              {
                "_count": "desc"
              },
              {
                "_key": "asc"
              }
            ]
          }
        }
      }
    }
  }
}

One result

{
  "key": "BRCA2",
  "doc_count": 18
}
}

When I set size to 20

{
  "key": "BRCA2",
  "doc_count": 21
}

Index Info:
Size: 2.5G
docs: 200w+
number_of_shards: 3

The expected quantity should be consistent

Steps to Reproduce

nested aggregations

Logs (if relevant)

No response