Why is box 21 on my T5008 form shown as income, when income is actually box 20 minus box 21? (I think)

In order to assist you, can you please give us more details. 

 

The calculation is box 21 minus box 20, and it will give you the total gain (or loss) you should be able to see on Schedule 3. As you can see on the following screenshot.

 

 

Thank you for choosing TurboTax.