First - I can not remember the last package we received that was shipped through Canada Post - in the last year was have seen something like half a dozen different companies at the door and I do not think any of them were Canada Post. About half the stuff was from the states.
If it wasn't UPS/Fedex/DHL, it was likely Canada Post handing the package off to a private delivery service. CP relies on these local delivery companies to help deliver packages across town, likely at a cheaper cost than hiring additional unionized CP delivery drivers. If the parcels were shipped USPS, then Canada Post would have almost certainly used a private delivery service to deliver the package to your door.
Many e-tailers who offer free shipping on orders over $xx.xx use Canada Post. I'm guessing they are quite a bit cheaper than Puro/FedEx/UPS.
Significantly cheaper due to the slower shipping speed, no expense for signature capture, no tracking and the option of slipping a pick-up slip into the door. UPS/Fedex/Puro/DHL expedite every shipment, have tracking, capture signatures and will try your address multiple times before requiring pick-up from the depot, and even then you can arrange another drop-off attempt.
UPS's basic Canadian rate is $22, even if the parcel is heading across town. To ship a small parcel via Canada Post across town the rate is about $8. As you tack on tracking and signature the price jumps significantly, but even then companies shipping lots of orders on a daily basis can benefit from CP's volume discounts.