I would think that it would depend on which ever carrier is the cheapest flying from your selected Airport at the desired departure time.
While I'm not speaking from experience, it seems like the only possible anwer because I don't think there is just one carrier that efficiently serves all the possible airports at the required times.
As an example, I'm booking my own air-fare for the Western Panama Canal cruise. Delta is the primary carrier in and out of our local airport (numerous flights thoughout the day). While there are other carriers, they have far fewer flights than Delta. So not suprising, the cheapest air fare to Florida is about $90 on Delta. Delta also has flights as low as $120 from LAX back to home, but they are only in the morning. Disney recomends you don't select departure times prior to 12:30 the day you get of the ship. All of the Delta afternoon flights are about $300. But United has flights that time of day for $160.