Turbotax was calculating an amount for the Canada Worker's Benefit (CWB), which is a refundable tax credit available to lower income workers.  However, you cannot claim the CWB if you were enrolled as a full-time student at a designated educational institution for a total of more than 13 weeks in the year, unless you had an eligible dependant at the end of the year.  As a result, once you entered your tuition receipt, Turbotax automatically removed this claim, lowering the refund.