Usually suff like this happens to stay competitive ie Air France puts on a seat sale just between YUL and YYZ and AC matches. Because AC has so many more options on getting to CDG it undercuts the fare out of YYZ if you are connecting through there.

If you book this and don't show up for the YUL flight it will cancel the rest of your reservation. If you book it and then try and call to cancel the YUL flight ahead of time they will recalculate the price and you won't be any farther ahead.