The net income from Canadian sources appears on line 1, which is a combination of employment income, benefits, and rental income. The rental income is then repeated on line 3, and the total minus rent appears on line 4, so the sum on line 5 is double my actual Canadian income.
Edit: it turns out you can edit this by going to the "Province" tax section on the left. It probably shouldn't have auto-populated with values, but at least I can fix it.