T3 Box 21 and Boxes 52 and 53

I am doing my mothers taxes and she has a T3 capital gain in box 21.

When I enter in boxes 52 and 53 ( which total BOX 21) the amount of tax owing goes up significantly.

I was under the impression that boxes 52 and 53 were included in box 21, and should not increase taxes owed if the dollar values are present.   It looks like TurboTax isn't using box 21 but the values in box 52 and 53.