it is common to purchase YYZ-YVR-SEA for much much less than YYZ-YVR and same for YVR-YYZ-ORD (or IAD or EWR)....and of course, not actually fly to the US. there is no need to purchase a flight out of SEA. of course it is also equally common to purchase very cheap last minute tickets from YYZ to JAX via ORD versus the very high last minute ORD flights...