Why does the addition of full-time student months reduce my refund? Isn't it supposed to be a credit that increases refund?

 Yes. It's actually quite common.

What's happening is that your income was low enough that you were assigned the Working Income Tax Benefit. Since students are excluded from this benefit, your tuition slip entry removed the benefit as it should.

It can be tempting to remove your tuition to receive that extra benefit but it's not a good idea. CRA's matching program will eventually discover the missing tuition slip and you'll be required to repay the benefit plus interest. It's just not worth it.

Thank you for choosing TurboTax.