Another possibility is the application by staff at the airport of the much wider discretion that they have to change things for you once you get there. Basically, they have the ability to change pretty much anything. As always, you cannot expect them to be willing to make any changes that the rules of your fare don't allow (and no doubt being difficult with them will reduce their willingness to help). But sometimes they can see things that you can't see that would justify them making a change for you that you also would like to have.