Why saveUninitialized is set to FALSE but the session still saved into database?

Question

I'm new to nodejs and express-session. As far as i know, when saveUninitialized is set to FALSE, the uninitialized session cookie will not be saved into sessionStore (mongodb in my case). Here is my session configs

session: {
  secret: 'tuxcoder',
  store: initializes.sessionStore, 
  resave: true,
  **saveUninitialized: false**, 
  cookie: {
    name: 'demo_app_cookie',
    maxAge: 60000, // 6 minutes
    secure: process.env.NODE_ENV !== 'production' ? false : true, 
    expires: 24 * 60 * 60 * 1000 * 7, // seven days\
    httpOnly: true
  }
}

In my database, the [sessions] collection is empty. I also delete all browser cookie. But everytime I make a request, there will be a session document in my collection.

{
    "_id": "WTN_EJhKTKnTQu5OPIpmhkIlTnJKVGKQ",
    "expires": ISODate("2024-04-05T05:56:00.346Z"),
    "session": {
        "cookie": {
            "originalMaxAge": NumberInt("604800000"),
            "expires": ISODate("2024-04-05T05:56:00.346Z"),
            "secure": false,
            "httpOnly": true,
            "domain": null,
            "path": "/",
            "sameSite": null
        },
        "flash": { }
    }
}

WHY?

Thanks for your time!