Self-Employment income is reduced by any self-employment expenses that you put in. The amount of taxes you owe is based on that net income, so it's possible that if you have enough in expenses, that you won't have any net self-employment income to tax. Or the amount of net self-employment income is so small that your credits, benefits & deductions (like your RRSP deduction) wipe out any taxes you might owe.
You can see you Net Self-Employment income on the detailed tax summary at line 13500.