YYZ-BKK is a routing based fare with specific cities that you can route through enroute to BKK. Neither PVG or PEK are one of these cities...which means the fare will always be broken and not priced as a through fare. NRT and HKG are the only cities eligible for this route.

You might be better off purchasing a YYZ-PEK/PVG ticket and a seperate to BKK....or just pay the higher price.

You could check other airlines as their routing rules might be different. Air China has a fare routing through JFK and PEK - YYZ-JFK on AA.