The amount of my combined T4 deductions that appears on my Turbo Tax return is less than the total amount deducted on my combined T4 slips.

If you are referring to CPP and EI deductions, are maximums that can be deducted in the year.  Anything in excess of this is reported as refundable credits.  Go to the Detailed Summary and look at lines 448 and 450 and see if the difference appears there.