I'm pretty sure this has been discussed before, but I can't find the thread. Sorry. (I looked -- I really did.) But I didn't understand the answer then, so I'm hoping someone can help me now.

I was looking last week at a flight from YYT to PHL via YYZ. My TA was looking at a Tango fare, and I was trying to help her find a Tango+ fare. (That's a post for another time, perhaps.)

I was looking at a Tango+ fare on the AC web site that I found by entering YYT to PHL. She was looking at just the YYT to YYZ portion and said there was no Tango+ fare showing. We went back and forth this way for a while, her saying it was not there, me saying it was. Then I checking the availability just for YYT to YYZ on the web site. Now I saw the same as her: no Tango+ fare showing.

I thought maybe it had gone while I was looking, so I searched again with YYT to PHL, and the Tango+ fare was back, for the exact same flight. I explained my results to the TA, and somehow that gave her what she needed to book it. So I got what I wanted.

But I'm totally confused. How can one flight be available at a certain fare class for one itinerary, and not for another? I thought AC did away with all (or at least most) of these shenanigans when they went to the one-way segment pricing model. Does anyone know what's going on here?