What may be happening is, in an effort to get you as many credits possible, TurboTax is automatically claiming the Working Income Tax Benefit. The WITB is a refundable tax credit meant to provide tax relief for low-income individuals and families already in the workforce, and to encourage other Canadians to enter the workforce. However, students are generally not eligible for this credit.
So if you tell TurboTax you had a low income, it will claim the WITB for you, and your refund will go up. But if you later add information that makes you ineligible for the WITB (ex: if you attended school full time for more than 13 weeks), you no longer qualify, and the credit will be removed from your refund.
To make sure that TurboTax knows that you don’t wish to apply for this credit:
Click “Find” and search for “Working”, then select “Working Income Tax Benefit” from the list and click on the “Go” button.
Select “No” next to the question “Do you want to see if you qualify for the Working Income Tax Benefit?”. Click the “Continue” button to save.