The theory was that AC was leading the price competition, and the PIT-YYZ segment was priced very low, because Porter was on the route. YYZ-PIT to this day is so much cheaper than YYZ-CLE. I did some dummy bookings and in FEB from CLE it's $600 r/t regardless of whether there is a Saturday night stay, on PIT it's $400 M-Fr or $300 Fr-Monday. My theory is that AC isn't able to pull in as much on the YYZ-PIT leg, but that completely fell apart when Porter went out. I know, I know, I know, everyone will tell me that airlines don't price one segment at a time, but what else could it be? There's absolutely no reason PIT should be any cheaper than CLE.
One thing I will add is that before Porter came along a close in PIT-YYZ weekend stay r/t was routinely $2,000 to ride in a clapped out 37 seat Dash 8. That's not a typo. When Porter came along AC naturally lowered their fares. When Porter pulled out (I still miss that airline) I fully expected AC to increase their fares to extortionist levels. But they didn't. They kept the 4x daily CRJs on the route and maintained reasonable fares. Perhaps they saw how the market was stimulated and want to maintain that customer base and market share.
One more observation about airline fares. It seems like there is a point where the added number of passengers attracted by bargain fares hits a wall. The airline will then jack up the fare to get the most out of a last minute must ride passenger. Take a look at DL's IND-CDG flight. I see round trips in April for $575 (IIRC they never offered fares that low from PIT). Probably because there are lots of empty seats so they are trying to stimulate demand to fill those seats. But lets look at a close in r/t, next Tue and returning a week later. Looking at the seating charts there are an incredible amount of open seats.... yet the lowest round trip is over $3,000. I'm just speculating here but I'd hazard to guess that those exact flights were substantially cheaper months ago to try to fill those cabins as much as they can. The pool of low yielding customers dried out so they are now selling for an insane amount to try to take advantage of the last minute passengers who must travel for whatever reason. They don't want to give away a r/t for $575 to that person.
I think trying to understand airline fares is much like predicting the weather. There is plenty of sound theory but the end result is often not what was forcast or is unpredictable.