Scanned keys for multikey indexes seem duplicated

In this simple example using a compound multikey index, how come the number of index keys examined is 4? I would have expected it to be 2.

Documents in my Collection:

        {
            "_id": {
                "$oid": "5f64582bb227df03421878f8"
            },
            "type": "app",
            "userId": "1",
            "myarray": ["1", "2", "3"]
        },
        {
            "_id": {
                "$oid": "5f645838b227df03421878f9"
            },
            "type": "app",
            "userId": "1",
            "myarray": ["4", "5", "6"]
        }

Index:

{userId: 1, myarray: 1}

The query:

{"userId": "1", myarray: {$in: ["1", "4"]}}

Explain output showing that 4 index keys were examined:

{
 "stage": "IXSCAN",
 "nReturned": 2,
 "executionTimeMillisEstimate": 0,
 "works": 4,
 "advanced": 2,
 "needTime": 1,
 "needYield": 0,
 "saveState": 0,
 "restoreState": 0,
 "isEOF": 1,
 "keyPattern": {
  "userId": 1,
  "myarray": 1
 },
 "indexName": "userId_mayarray",
 "isMultiKey": true,
 "multiKeyPaths": {
  "userId": [],
  "myarray": [
   "myarray"
  ]
 },
 "isUnique": false,
 "isSparse": false,
 "isPartial": false,
 "indexVersion": 2,
 "direction": "forward",
 "indexBounds": {
  "userId": [
   "[\"1\", \"1\"]"
  ],
  "myarray": [
   "[\"1\", \"1\"]",
   "[\"4\", \"4\"]"
  ]
 },
 "keysExamined": 4,
 "seeks": 2,
 "dupsTested": 2,
 "dupsDropped": 0,
 "parentName": "FETCH"
}

Hi @Elina_Larsson,

Refer to the following documentation to understand how index keys are examined:

If you still have any questions, I would recommend you to post your query in our community forum for a more involved discussion.

Kind Regards,
Sonali