That YHZ-YQY ripoff – a partial explanation

I am posting from the tarmac at Montréal-Trudeau  Airport, part way through the strangely priced Air Canada flight I wrote about here. Contrarian reader Joe MacKay offers a plausible if partial explanation for Air Canada's charging more for a Halifax-Sydney ticket than the Toronto-Sydney ticket I'm flying on, even though the Halifax-Sydney leg is the same flight on the same plane I'll be taking. I think this was a side effect of a Porter sale. Porter ran 50% off flights from the Island briefly a week or so ago. Air Canada responded (as they do) with a predatory sale on all bookings...