[gpt5]

This is because sometimes the airliner sell a flight connecting through your departure airport for less than a flight departing from it.

Let's say you want to fly from Vegas to New york, it might be cheaper to buy SFO to new york, and hop on the Vegas->Mew York leg. They essentially make this option less viable, and can price each market differently.

[havblue]

That makes sense. For me, what happened was I was scheduled to fly to a destination city but wound up flying there from a different departure city, earlier, due to a funeral. I was shocked to find out that they would have cancelled my return trip as well if I didn't call in to grovel.