And here I thought that they used the airlines becuase they were recognized names. United Airlines = United States. American Airlines = Americans. Anyway I don't know why people give security people any shit at the airports. They are government hired and paid by the hour, so you can bitch at them all you want. They will just make life worse for you.
As for the actual questions, I would guess Terrorists do several dry runs, and the ones that they can actually fullfill their operation on, then they use those. The ones they have trouble on, or get asked too many questions they stay away from.