Any item with a declared value of more than C$20 is subject to taxes (PST / GST / HST) and duty, if applicable. Some items don't even have this C$20-threshold, such as books.
http://www.cbsa-asfc.gc.ca/import/postal-postale/duty-droits-eng.html
If you receive an item with a declared value of more than C$20 in the mail and Canada Border Services hasn't taxed it and Canada Post hasn't added another C$9.95 in processing charges, it's more due to the luck of the draw and the lack of economic sense to tax it.
Unlike Canada Border Services, the Global Shipping Program isn't "allowed" to let taxable items slip through untaxed.