It is a CRA rule on how to use the schedule 11. You have to use the unused tuition amount, then the current tuition amount to reduced the non-refundable tax credits to zero. If you have any extra amounts you can carry forward or transfer to other tax-payers.
You cannot override this amount on schedule 11 or T1-General, and if you try your return will be post-assessed by CRA.
I hope this was helpful
