Just because they publish a S class fare from LAX-YUL doesn't mean they are willing to sell it on the direct flight. So they control the available S seats. However, if you are willing to route via YYZ,YYC,ORD,IAH etc etc you might find the S space you want when purchasing via AC.com

The YUL-LAX // LAX-YUL is often very very busy. V class is often all we can get on our contract fares for this route.