The reasons for these discrepancies may vary and can range from simple (typo or data entry error, forgotten slip, receipt, amount, etc) to complex issues (carry-forward amounts not included, etc).
The CRA collects and maintains your tax information, and they may already have some of your tax slips for the current year (for example, T4 slips submitted to the CRA by your employer) as well as other tax-related information from previous years (such as carry-forward amounts, RRSP limits, etc.). The CRA considers all of this information in totality when making the assessment on your tax return.
In most cases, the CRA will explain how they calculated your tax refund or balance owing, and you can find this information on your Notice of Assessment (NOA), under the heading Explanation of changes and other important information. To find the discrepancies, please compare the line numbers on your NOA and their corresponding values with those entered into TurboTax. If you used the TurboTax online edition, please use the printed or PDF copy of the tax return you filed using TurboTax or the detailed summary page of your tax return to do the comparison.
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