In most cases the Canada Revenue Agency (CRA) will explain how they calculated the amount of your refund/balance owing. You will find this information on your Notice of Assessment (NoA). You can compare the line numbers on your NoA with your tax return to see what the CRA changed.
There may be different reasons for the discrepancies in the calculations (typo or data entry error, forgotten slip, receipt, amount, carry-forward amounts not included, accounting errors, etc).
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