Why do Database Generated Values create columns when their values are always default?

Hi, 

I have a model and one of the properties is a calculation based on another property (average age).

Below is just pseudo code and I have not tested it. 

public class Person
{
    public int Id { get; set; }
    public string FirstName  { get; set; }
    public string LastName { get; set; }
    public int Age { get; set; }  

    [DatabaseGenerated(DataGeneratedOption.Computed)]
    public string FullName
    { 
        get { return FirstName + " " + LastName; }
        private set { }
    }
}

public class Students
{
    public int Id { get; set; }
    ....
    public virtual ICollection<Person> People { get; set; }


    [DatabaseGenerated(DatabaseGeneratedOption.Computed)]
    public int AverageAge
    {
        get { return People.Sum(p=>p.Age) / People.Count; }
        private set {}
    }

}

In another similar example that I tried, the above would generate columns for both FullName and AverageAge, with default values.

Now from my understanding of this line [DatabaseGenerated(DatabaseGeneratedOption.Computed)], the values are re-calculated when a row is added or updated - which is great for my usecase. 

My questions:

1. If all the values generated via `Identity` or `Computed` are not set in the application, but rather, the database is responsible, then why does the database even create a new column considering these columns only seem to contain identical, default values? 
2. With the assumption above, what happens if you add the attribute [NotMapped] to those properties? The column would not get generated, but does the calculation still occur?