mystery on why compute oonly calculates for "Total" column

[sid alam]

I have an employer table with the following columns
employer
DepartNo | EmployerName | Salary | Commission | Job
How comes I am not able to calculate total for salary and commision but it calculates the total(sum) for "Total"???

col my_column1 HEADING 'Monthly Salary'
col my_column2 HEADING 'Annual Commission'
BREAK on DepartNo skip 1
compute SUM OF my_column1, my_column2, total ON DepartNo
select DepartNo , Job "Job", employerName "Name", Salary my_column1 , Commission my_column2, sal*12+(nvl(comm,0)) "Total" from employer
order by deptno;