Originally Posted by LGAflyer

Well the reverse is not the same thing as the return appears to be a 'sterile' transfer. When you arrive at YYZ from international, you only go to customs B if youre connecting to international. But in this hypothetical case you would be going transborder so you go to customs A then stay to the left. You never proceed through the CBSA checkpoint but rather go up an escalator that drops you inside the transborder connecting luggage area to proceed through CBP checkpoint.