Have had some very helpful and detailed PMs from someone in the AF-Cityjet know (don't know if said person is "known" on proon in that capacity so will reserve his/her identity
) who pointed out that that a same day SNN-CDG-YYZ can't be made as the connections are optimised for east bound rather than west bound, which given SNN's west bound connections is understandable.
The expedia price I saw earlier turns out to be an
overnight connection at CDG. That's one booking engine quirk I've long wished could be turned off on ba.com.
He/she did point me towards the open jaw YYZ-CDG-SNN/DUB-CDG-YYZ which is priced comparably.