GORM create from map

I'm learning gorm through the official doc: https://gorm.io/docs/create.html
When coding the demo of Create from map, I can't understand the meaning of Note

When creating from map, ... primary key values won’t be back filled

But I still can find three records in my mysql which primary key values are back filled.
This is my code segment:

type User struct {
    gorm.Model
    Name         string
    Email        *string
    Age          uint8
    Birthday     time.Time
    MemberNumber sql.NullString
    ActivateAt   sql.NullTime
}
...

func createFromMap() {
    db.Model(&User{}).Create(map[string]interface{}{
    "Name": "jinZhu11", "Age": 18,
    })

    db.Model(&User{}).Create([]map[string]interface{}{
    {"Name": "jinZhu8", "Age": 18},
    {"Name": "jinZhu12", "Age": 20},
    })
}

The MySQL records：