TurboTax does guarantee our calculations to be 100% correct. You should first compare your Notice of Assessment (NOA) with the PDF file of your TurboTax return you filed and see where the discrepancy is. (Your NOA may also tell you if there was a change made to what you filed). 

Often times there is an extra tax slip out there (T4 from a little job you missed, or T4E for EI benefits you thought were paid in the prior year), and CRA will add this to your tax return when they assess it. Or if you had a previous "balance due", CRA will add that to this year's amount due.

If you cannot determine from your NOA and TurboTax comparison where the difference lies, contact CRA and ask for an explanation. 1-800-959-8281.

If you do feel that this a calculation error on TurboTax's part, please contact us and our guarantee will apply. Here's how to go about doing so: https://turbotax.community.intuit.ca/replies/2647678